Synchronos Dynamic Random Access Memory.
You could say it's more in 'synch' with the cpus timing rather then some other clock controlling it like how EDO(Extended Data Out) or FPM(Fast Page Mode) worked. - I think
DDR-SDRAM is Double Data Rate is basically regular SDRAM desgined to send data on the rise and falling edge of a refresh cycle. -
RDRAM(don't know the meaning of it, but guess is Rambus Dynamic Random Acccess Memory) is the lame type of memory that the P4 was designed around. Rambus is in legal trouble because it tried throwing it's weight around recently by trying to get DDR vendors to pay them royalties for the DDR spec, which they did not rightfully own.
Found this page here:
Hmmm, looks okay.