a page is made up of many different html elements. each element can have a single ID or multiple classes associated with. any css that is applied to an element (like a div) applies to all it's children also. if you want a child element to have a different setting, you must code a CSS selector pointing at it and the rule to change the setting.
Firebug is an extension to FireFox that allows you to examine teh HTML on a page and teh CSS that applies to it.
If this is greek, then you need to take a CSS tutorial. A good on can be found at
http://htmldog.com/guides/css/