Basically, it's speed, pin structure, and bandwidth.
PC100 - 164 pin DIMM, meant to run at a front side bus speed of 100 MhZ.
PC133 - 164 pin DIMM, meant to run at a FSB of 133 MhZ.
DDR RAM - 184 pin Dimms. These run at either 100 Mhx (PC 1600) or 133 MhZ (PC2100), but they transmit...