newbie help

[psychotic]

Ok, I understand that a rom is an altered, customized version of the Windows Mobile interface....

Being brand new to pocket PCs, I could use a tiny help in the following:

I have WM 5.1.195 build 14847.2.0.0

I know I should probably upgrade it to build 2.06.00

here is what I don't understand....
What does AKU stand for? what is AKU 3.5?
Is windows mobile latest build still 2.06? I do not understand the 3.3, 3.5 etc rating that you guys use when building custom roms.
Also, extended rom, I have no clue what that is for nor what it is exactly......
Ive read over the noob guids and wiki, it helped... somewhat, but i'm still a little lost. hopefully a few answers will point me in the right direction

and, Hello everybody!
-Ben