DIMM
Dual inline memory module. A module with signal and power pins on both sides of the board (front and back).

Buffered Memory
A buffer isolates the memory from the controller to minimize the load on the chip set. It is typically used when the system has a high density of memory and/or when a system has more than 3 memory module sockets.

Unbuffered memory
This is where the chip set controller deals directly with the memory. There is nothing between the chip set and the memory chips on the module as they communicate.