Function Key?

Hello,
What does the Fn for function key DO?
Also, tell me about top row on keyboard (F1 to F12)? What are these keys for?
Also: what do the following keys do and mean? Translate, please:
Ins/Prt Sc
SysRq
Scr Lk
and on bottom row of keyboard:
Alt and Alt gr?
The picture on the key next to Alt Gr looks like a printed page?
The icon on the left with four squares, like a flag?
I need a manual really badly!
WHY NO MANUAL explaining all these mysterious keys?

Thank you,
BJ