Ah, I see you have a question about CSS. Yes, classes are used in CSS, Use Chrome, and right click on the element and choose inspect element.
This should pull up the developer tools. If you need to determine what CSS file is controlling this class, you might need to turn off "Use speed-up tool for CSS" in the general settings section of the admin. Then you can see exactly what CSS file is taking priority.
Good luck, let us know if you have any more questions.
Here is a decent explanation of classes:
http://htmldog.com/guides/css/intermediate/classid/