/me turns on sirens and lights on his "Ignorance Police" vehicle.
Ok, there are several types of RAM . .
There was old Static or SRAM, Extended Data Out or EDO, DRAM or Asynchronius Dynamic RAM, SDRAM or Synchronius Dynamic Ram, SDRAM DDR, RDRAM or Rambus and DDR2.
DDR stands for Double Data Rate, and it's ram can refresh twice per clock cycle. Old SDRAM can only refresh once. At the same clock speed, DDR is twice as fast. DDR2 is simply a better design which allows for higher clock frequencies with cleaner spectrums.
Rambus was the fastest, but it costs an arm and a leg. DDR SDRAM is the most popular in production, soon to be replaced by DDR2. SDRAM itself is still usefull in cheap business PCs (PC66, PC100, PC133).
Oops, forgot Dual Channel DDR . . . almost the same as regular DDR, only it has extra traces for doubling bandwidth while using up more ram.