IDs are similar to classes, except once a specific id has been declared it cannot be used again within the same (X)HTML file. I generally use IDs to style the layout elements of a page that will only be needed once, whereas I use classes to style text and such that may be declared multiple [...]

The class selector allows you to style items within the same (X)HTML element differently. Similiar to what I mentioned in the introduction about inline styles. Except with classes the style can be overwritten by changing out stylesheets. You can use the same class selector again and again within an (X)HTML file. To put it more [...]