SDRAM - Synchronous Dynamic Random Access Memory
Synchronous DRAM is a type of DRAM which is an improvement over conventional DRAM. It can run at much higher clock speeds (at 133 Mhz) than other types of RAM. The name synchronous has been added before it, because it synchronizes itself with the CPU's bus speed. So, the faster the bus speed, the faster the SDRAM could be.
The synchronous mechanism of SDRAM is driven by the computer's clock. Once the clock sends the signal saying another unit of time has been passed, the memory chip starts working. In this way it gets synchronized with the computer's clock.
The speed of SDRAM is measured in terms of "Mhz" rather than in nanoseconds (ns). This makes it easier to compare the speed of chip with bus speed.