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 data on both the up and down sides of the clock cycles. This means that PC2100 DDR (Running at 133 Mhz) should perform like regular 164 pin DIMMs at 266 MhZ.
RAMBUS (RDRAM) - Patented and more expensive RAM, used exclusively in Intel systems. Very high speed and bandwidth, but performance doesn't necessarily justify the price tag, and the need to go Intel (Personal opinion on that last).
PC100 was the first really fast stuff to come out, and was quickly followed by PC133, which was the defacto standard for awhile.
Then Intel decided to go almost exclusively RDRAM, and DDR was brought out as a lower cost competitor. AMD is using DDR in many of its newer configurations, and Intel is now looking to go the same route. DDR has been used in high end graphics cards for quite some time now.
Bored yet?