I've never really understood the difference between class and ID (. & #).

From what I've read you should use ID when you only use that style once and class if you use it multiple times but that doesn't seem right to me.

Can anyone clear this up for me?

Thanks.

An ID may only appear once on the page. Only one ID may be used per element. A selector with an ID is higher in the Cascade than a Class in a CSS document, in other words, it has greater specificity by default than a class.

A class, on the other hand, may be used multiple times on a single element, and may appear multiple times throughout the page.

An ID is for unique elements. A class is for generic, reusable elements.

Just what Abel said. An id should be used once and only once per page and a class can be used as many times as you want. You can use the same id on different pages though. Each page can have <div id="header"> since each page will likely have a header. However no single page would have more than one header.

Sometimes your choice can seem a bit arbitrary. For example you could make your header a class <div class="header"> and then use it only once on the page, but technically you should use an id when something is only meant to be used once per page and use classes for things that can be used more than once, even if you only end up using them once per page.