how do you use a CLASS ???

ok, this must be the simplest question alive, but help me please?? i've seen loads of classes all over the net, and i want to use them, but not sure where to start! are class files independent of platform?? do you have to name your variables same as them? or do i just call it or something? they dont usually have documentation, so i'm a bit stumped :( /sunny ____________________________________________________________ Do You Yahoo!? Get your free @yahoo.co.uk address at http://mail.yahoo.co.uk or your free @yahoo.ie address at http://mail.yahoo.ie