Please help me, I'm having alot of trouble understanding the concept of emulation of multiple memory mappers. I have some ideas on how to do it, I just can't find much info on how these devices work, and that is making it very hard to understand how to emulate them. I can't do much with just the mapper info (ie. the registers, and what it does) I also need to know how these devices are used in conjunction with the NES, how the CPU accesses them, and their registers.

Open this: http://nesdev.parodius.com/mappers.zip It contains a file that has extensive documentation on the registers and functions of many, many different mappers.
Apparently the 6502 itself plays no role in memory mapping except to write values to the mapper registers (which are located in the ROM space ($8000-$FFFF)); I presume once the value(s) has/have been written, the mapper and the 6502 go on their own merry ways.

I think the mappers' primary role is to control what the CPU has access to; think of them as a switch at a fork in a railroad track. The word "gate" also comes to mind. I can't think of a better way to explain it.
*shrugs* I hope that document helps you out.

With mappers, you can see a part of the main memory as a "window", it only show a portion of the program there. Writting to $8000 and up just activate the mappers features. If you were coding in the good old DOS age and used EMS memory, it's must the same like mappers, except that EMS was using RAM memory...

.ORG $8000
LDA #$7F
PHA
LDA #$FF
PHA
RTS