When creating a web site, when should one use a Class, and when should they use an ID? Both are great ways to distinguish elements that may appear repeatedly throughout an html document.
ID's are identifiers that are unique. They are special in this way. You should not reuse ID's when creating your html document. One reason is that it will cause issues when you try to validate your code. Also, in some instances, it will be helpful like when you are searching for it on a page.
Classes on the other hand are not unique and can be used mulitple times. If you know you can blanket a certain collection of styles into many different places on your html document, using classes can drastically speed up your coding. You wouldn't have to repeat your styling constantly. This also helps in keeping your code DRY.