Questions of rom sets and hack/bootleg history

[reebboy]

I have a few questions about rom sets in mame.  First what is the difference between sets and revisions in a particular rom set. Example Marble Madnes has set  1, set 2 etc. Is this the same as a revision?? Also why does it seem like Atari had numerous revisions for many of their earlier games. Example APB had 7 revisions.  Were these revisons very different from each other or did the earlier revisions have bugs that kept needing to be corrected?

Last question, Is there a website that explains the history behind many of the hacks and bootlegs of many roms. How and why they they came about? 

[itismejs]

They made bootlegs to make money...