CLASS Question

I am new to scripting and just getting started on an AutoIt tutorial. The tutorial does not mention CLASS, but the examples in the AutoIt help show CLASS as in:

WinActivate("[CLASS:Notepad]", "")

How does CLASS work and how do I get more info on this? I have searched the help files and the internet, but I still don't understand how this works.

One more question, is there a way to search these forums?

Thanks.