What's the difference between an id and a class?

Question

ids must be unique where as class can be applied to many things. In CSS, ids look like #elementID and class elements look like .someClass

In general, use id whenever you want to refer to a specific element and class when you have a number of things that are all alike. For instance, common id elements are things like header, footer, sidebar. Common class elements are things like highlight or external-link.

It's a good idea to read up on the cascade and understand the precedence assigned to various selectors: http://www.w3.org/TR/CSS2/cascade.html

The most basic precedence you should understand, however, is that id selectors take precedence over class selectors. If you had this:

<p id="intro" class="foo">Hello!</p>

and:

#intro { color: red }
.foo { color: blue }

The text would be red because the id selector takes precedence over the class selector.

Perhaps an analogy will help understanding the difference:

<student id="JonathanSampson" class="Biology Calculus" />
<student id="MarySmith" class="Biology Networking" />

Student ID cards are distinct. No two students on campus will have the same student ID card. However, many students can and will share at least one Class with each other.

It's okay to put multiple students under one Class title, such as Biology. But it's never acceptable to put multiple students under one student ID.

When giving Rules over the school intercom system, you can give Rules to a Class:

"Tomorrow, all students are to wear a red shirt to Biology class."

.Biology {
  color: red;
}

Or you can give rules to a Specific Student, by calling his unique ID:

"Jonathan Sampson is to wear a green shirt tomorrow."

#JonathanSampson {
  color: green;
}

In this case, Jonathan Sampson is receiving two commands: one as a student in the Biology class, and another as a direct requirement. Because Jonathan was told directly, via the id attribute, to wear a green shirt, he will disregard the earlier request to wear a red shirt.

The more specific selectors win.

Where to use an ID versus a class

The simple difference between the two is that while a class can be used repeatedly on a page, an ID must only be used once per page. Therefore, it is appropriate to use an ID on the div element that is marking up the main content on the page, as there will only be one main content section. In contrast, you must use a class to set up alternating row colors on a table, as they are by definition going to be used more than once.

IDs are an incredibly powerful tool. An element with an ID can be the target of a piece of JavaScript that manipulates the element or its contents in some way. The ID attribute can be used as the target of an internal link, replacing anchor tags with name attributes. Finally, if you make your IDs clear and logical, they can serve as a sort of “self documentation” within the document. For example, you do not necessarily need to add a comment before a block stating that a block of code will contain the main content if the opening tag of the block has an ID of, say, "main", "header", "footer", etc.

An id must be unique in the whole page.

A class may apply to many elements.

Sometimes, it's a good idea to use ids.

In a page, you usually have one footer, one header...

Then the footer may be into a div with an id

<div id="footer" class="...">

and still have a class

A CLASS should be used for multiple elements that you want the same styling for. An ID should be for a unique element. See this tutorial.

You should refer to the W3C standards if you want to be a strict conformist, or if you want your pages to be validated to the standards.

IDs are unique. Classes aren't. Elements can also have multiple classes. Also classes can be dynamically added and removed to an element.

Anywhere you can use an ID you could use a class instead. The reverse is not true.

The convention seems to be to use IDs for page elements that are on every page (like "navbar" or "menu") and classes for everything else but this is only convention and you'll find wide variance in usage.

One other difference is that for form input elements, the <label> element references a field by ID so you need to use IDs if you're going to use <label>. is an accessibility thing and you really should use it.

In years gone by IDs were also preferred because they're easily accessible in Javascript (getElementById). With the advent of jQuery and other Javascript frameworks this is pretty much a non-issue now.

Classes are like categories. Many HTML elements can belong to a class, and an HTML element can have more than one class. Classes are used to apply general styles or styles that can be applied across multiple HTML elements.

IDs are identifiers. They're unique; no one else is allowed to have that same ID. IDs are used to apply unique styles to an HTML element.

I use IDs and classes in this fashion:

<div id="header">
  <h1>I am a header!</h1>
  <p>I am subtext for a header!</p>
</div>
<div id="content">
  <div class="section">
    <p>I am a section!</p>
  </div>
  <div class="section special">
    <p>I am a section!</p>
  </div>
  <div class="section">
    <p>I am a section!</p>
  </div>
</div>

In this example, the header and content sections can be styled via #header and #content. Each section of the content can be applied a common style through #content .section. Just for kicks, I added a "special" class for the middle section. Suppose you wanted a particular section to have a special styling. This can be achieved with the .special class, yet the section still inherits the common styles from #content .section.

When I do JavaScript or CSS development, I typically use IDs to access/manipulate a very specific HTML element, and I use classes to access/apply styles to a broad range of elements.