id vs class

Tristan,

Use IDs when you're describing something unique--for example, a section that appears only once. Use classes when you're describing something you might use again.

Here's a good article on the subject.

http://css-tricks.com/the-difference-between-id-and-class/

An id refers to a specific object in the document object model and is unique to that one object.

A class is re-usable with many objects on a page.

You may have a portfolio of images. The <ul> containing the list of images may have an id of "portfolio-images" because it only exists once on the page. The <li> tags in the list may have a class of "image-container" to define each image in the list.

<ul id="portfolio-images">
    <li class="image-container"><img src="../someimage.jpg"></li>
    <li class="image-container"><img src="../someimage.jpg"></li>
    <li class="image-container"><img src="../someimage.jpg"></li>
    <li class="image-container"><img src="../someimage.jpg"></li>
    <li class="image-container"><img src="../someimage.jpg"></li>
    <li class="image-container"><img src="../someimage.jpg"></li>
</ul>