This is a common question on css- ID vs Class.
The key thing to know is that IDs identify a specific element and therefore must be unique on the page - you can only use a specific ID once per document. Many browsers do not enforce this rule but it is a basic rule of HTML/XHTML and should be observed. Classes mark elements as members of a group and can be used multiple times, so if you want to define a style which will be applied to multiple elements you should use a class instead.
Also, an element can have multiple class values (by space-separating the various values), while an ID must be a single value. In this way, a single element can "inherit" from multiple classes.
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