One of the most essential components that is plugged in to your motherboard is your system's main memory. There are essentially three types of memory (or "RAM") that you will find for your motherboard: DDR / SDRAM, RDRAM, and SDRAM.
Double-Data Rate RAM essentially doubles the rate of standard SDRAM. Any DDR RAM will be quicker (and probably more expensive) than its non-DDR counterpart.
RDRAM basically doubles the output of SDRAM and also moves the data quicker. This is a high-end variant of RAM.
This is a high performance, faster variant of some other types of RAM. The S stands for "synchronous" because it uses a new technology to send spurts of data at specific intervals of time.