Stands for "Synchronous Dynamic Random Access Memory." Yeah, it's a mouthful, but if you memorize it, you can really impress your friends. SDRAM is an improvement to standard DRAM because it retrieves data alternately between two sets of memory. This eliminates the delay caused when one bank of memory addresses is shut down while another is prepared for reading.
It is called "Synchronous" DRAM because the memory is synchronized with the clock speed that the computer's CPU bus speed is optimized for. The faster the bus speed, the faster the SDRAM can be. SDRAM speed is measured in Megahertz, which makes it easy to compare the processor's bus speed to the speed of the memory.