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Should I be using classes more than ID's when utilizing jQuery?

As a new web developer, I've been utilizing a lot of resources like StackOverflow to assist me in the learning and development process.
When using jQuery, all of the examples/responses that I've come across so far have only referenced classes, like so:
$('.yourClass')
as opposed to
$('#yourID')
Seeing that class referencing seems to be the trend (I honestly haven't found one author who writes a jQuery to an ID), are there any pitfalls I should be aware of for using ID's w/ jQuery or JS in general? Thanks!
EDIT 1: I'm aware that ID's are for single-items, classes are for accessing multiple items. I'm more interested in why I don't see any jQuery or JS examples referencing ID's. Thank you!

You would have to ask each author on a case-by-case basis, but generally when creating examples, the selector used doesn't matter; what's important is that you have a jQuery collection that you can call a method on.
By using a class selector in the example, you avoid newbie developers claiming that your plugin doesn't work when they try to use it on multiple elements with the same ID. Your example serves the purpose of showing how to use it on one or more elements, rather than just one.

People like to use classes because ids have to be unique across the whole page. When trying to make reusable, pluggable components, id's make this impossible to enforce.
Exception: the new web-components standard allows you to encapsulate ids to just your component.

An ID must be unique, you can have only one (like highlanders).
Classes are used to identify a "type" of object not a specific one.
An obligatory car analogy:
An ID is a license plate, unique to one specific thing #345-abc
The class relates to a whole category of things like .truck
Take note that a selector like $(".something") will actually be capable of producing a list of DOM elements; as it will select all DOM elements with the class of "something"
An ID selector $("#unique") will only ever return one element

Think of your HTML and CSS first.
Using Classes
If you have multiple HTML elements which all will look, feel and behave in the same way, then it is highly recommended to use a class to represent their style and behavior.
Example: rows or columns on a table, navigation buttons which animate in the exact same way, wrapper to images which have the same size throughout your website, etc.
Using ID's
However, if you have a unique HTML element which represents a particular thing or state or action in one of your pages, then that element should contain an id.
Example: pop up modal, a unique looking button, unique sections on your website which you can navigate to by their id, etc.
Then, you can use this behavior in your JavaScript and jQuery or whatever else you like to use.
Further reading

I know that you are fully aware of why we should use ID's or classes.
But the vast majority of answers that are given here, are thinking of a project context.
So, let's say editing a .js file that is linked to the scope of the entire project, the idea here is to be as reusable as possible, so that's why you'll see much more classes references than ID's. Is hard to maintain a project js file that makes reference to different ID's that are abroad the project.
Same thing will apply to css.
I hope the answer is enough, be free to post a comment or suggestions. :-)

Why does jQuery Mobile use data-role attributes instead of classes?

This may be a naive question, but I'm learning jQuery Mobile and unsure why everything is related to a data-role attribute. It seems that even times when the role is related to the style, they are using data-role over class
I understand that for semantics, data-role makes sense in a lot of cases but it just seems to be abused here. Wouldn't that kind of lookup also be slower?
Why do they use data-role over class?
Thanks for any insight.

Why data
They could simply use role as an attribute (like other frameworks do), but this would make
the resulting HTML invalid. Therefore a data- is added to every attribute name.
Why not classes
I think the main reason for that is to separate view from logic as far as it is possible. In larger projects, CSS and JavaScript are not written by the same person.

It provides a lot of control over powerful styling techniques especially when combined with jquery ui. I use jquery mobile, I used their tool to easily make a theme roller and now when I use elements like data-role-header, footer listview. I have great looking pages with no effort. There are hundreds of data-role attributes you can bring into your code to easily create uniform, user friendly pages. I personally like the data-role - page attribute to create multiple views in a single HTML page. They are easy to use so the best way to learn about them is to play with them.

Please find the explanation of data-roles here.
data-role attribute is used to control the behaviour of the widget of element. For example in case of button you can use input type="button" (no data-role="button" attribute required in this case, as this is standard behaviour of this element) but you can use a element, and then you need to explicitly provide it:
So for me it's rather useful solution, as buttons behavior on mobile devices can be same for different elements. You just need to provide data-role attribute, and jQuery will do the rest for you.
This is the quotation from main jQuery Mobile website:
jQuery mobile framework takes the "write less, do more" mantra to the
next level: Instead of writing unique apps for each mobile device or
OS, the jQuery mobile framework allows you to design a single
highly-branded web site or application that will work on all popular
smartphone, tablet, and desktop platforms.
They want to style every control you have in the same way, so write less, do more approach is fulfilled. So jQuery Mobile adds same styling for all elements with the same role to make things look the same way, but it doesn't mean you can't change it. It just means that they care about good look of your website, and they are aware that every button should be similar to others.
Also the page I mentioned earlier says:
The jQuery Mobile framework uses HTML5 data- attributes to allow for
markup-based initialization and configuration of widgets.
So you are reading HTML and you know how elements will behave without looking to CSS file - which I think is cool if you're not front-end dev. Of course front-end dev can overwrite CSS, but he must follow the rules, e.g. if data-inline is set to true he should style it regarding that elements must naturally follow this rule (be inline).

jQueryMobile adds a load event handler to the page, which processes the DOM looking for various data-xxx attributes. When it finds those, it does more than just stylize the elements.
In many cases it creates a type of widget tied to the data-role. For example, a <div data-role="header"> is turned into a toolbar widget, the creation of which may extensively modify the DOM within that element.
For some of the simpler widgets, like buttons, folks have seen that not much happens other than some classes get added, so why not just shortcut the process and do that directly? That can work, but it isn't future-proof. At different points in history, different versions of jQM had created buttons with different DOM structures. So I personally think it's best not to shortcut jQM, and let it process the data-attributes as it sees fit.
That being said, it would still have been possible to create widgets identified by classes rather than data-attributes, which was how people used to do these things before jQM. But then there might be an expectation that there would be CSS associated with those classes as well. Use of the data- attributes makes it clear that this is a structural/role thing rather than just styling.

Set variable to HTML elements with certain class?

I'm sure there is a way to set a JavaScript variable to any HTML element with a certain class attached to it? I'm just not sure how to write it. Can anyone help me out?

The best way would be to use a framework, such as jquery, that makes an easy use of (css) selectors, e.g, to select all elements with class my-class do
$('.my-class')
and then apply any code to the list of those elements
edit: don't forget to use the $(document).ready wrapper

you could simply go var x = document.getElementById('somediv'); for example. Now whatever style class pertains to somediv will of course still obtain unless you change it programatically in your javascript.
Hope that helps -- might be able to be more helpful if you give a broader context of what you are trying to accomplish.

To do this without document.getElementsByClassName('myclass') nor $(.'myclass') would be difficult and require some advanced tactics. You would have to recursively generate a list of all DOM objects from the document. While generating the list, every element touched would have to be tested for .className = 'myclass'. This is essentially what jquery does behind the scenes, although I believe on page load it caches the whole DOM for easier querying.

A convention for indicating whether an HTML element is referenced from JS code

This is a follow-up question for In jQuery is it a bad idea to use name=X for all selectors?
I am using Backbone and decided that I wanted a way to differentiate between HTML elements that were bound and those that were not.
So I would write (in HAML):
.container
.title(name='title')
.separator
As you can see it's clear that the dynamic element is title.
The reason for this was so I could mess around with the style and rename classes without worrying about breaking the app. It also means in the template I can tell what the dynamic elements are without needing to go back and forth with the Backbone View.
My question now is, without using the [name] selector, does anyone have a code convention to keep track of which HTML elements are referenced from JS.
I have considering:
Using a common prefix on class names (e.g. class=bind-title)
Using some sort of custom HTML element (
Thanks!
FYI: I'm using CoffeeScript, Backbone and haml_coffee templates.
Updated jsperf to test all suggestions:
http://jsperf.com/class-or-name-attr-lookup/3

I would consider using a class to indicate that it is dynamic.
I'm not sure if you are aware of this but you can have multiple classes on one element. Like so:
.container
.dynamic.title(name='title')
.separator
This works in traditional HAML but I have not tried it with haml-coffee. If it doesn't work, you might have to specify the class like .title{:class => "dynamic"}(name='title').
I prefer this over a prefix on the class name because it's more semantically meaningful, which is how HTML should be used.

I am using data-view attribute on elements being set when rendering my Views.
This helps me to then show a tooltip in a browser window when I hover over View(s).

Is it wrong to put ID's on DOM elements for automation because it would turn their CSS classes into singletons?

I'm wondering what the correct way to go about Automation (like Selenium) is. I was told that I shouldn't put ID's on elements, because this can lead to JS errors (if duplicate ID's exists) and can cause an elements CSS classes to become singletons. I would agree with this, but not having IDs can make Automation a pain in the rear.
Thoughts?

You got some pretty bad advice.
The "id" and "class" namespaces are distinct. Give page elements (unique) "id" values when you need to find them efficiently and reliably.
(What you mean by "Automation", by the way, is not entirely clear.)

you should still use ID's for unique identification JS-wise, but should style for your classes CSS-wise
<DIV id='DIV1' class='basicDiv' />

I think a better rule of thumb is to be careful when putting IDs on elements. This is true for virtually all languages, the problem with non-compiled languages is that depending on your editing environment this won't be discovered until run-time. The browser will not complain about HTML elements with duplicate IDs until you try to reference it in JavaScript.
IDs are very useful in unique identifying sections of your HTML, but are used differently than classes. Usually, classes are used to style elements that share common properties while IDs are used to uniquely identify a specific element for processing.