Hello,

If you need a quick introduction to UML class diagram I suggest taking a look at the following article:
UML basics: The class diagram. You can also check Wikipedia for a quick overview of UML here.

If you want a deeper knowledge to UML then I would suggest that you pick a book, there are plenty
of them out there (such as this one by Martin Fowler).

Also if you want the nitty gritty details I would suggest the UML specification, but it's a heavy read :o)

Good Luck!