DDR is Double Data Rate RAM.
RAM is computer memory that is needed for a CPU to have quick access to information on the PC.
DDR is the original.
DDR2 is faster and is still widely used but slowly being replaced by DDR3.
DDR3 is faster than DDR2 and is the newest industry standard.
You can usually tell by looking at the memory modules themselves. The easiest was is by looking for a sticker on the RAM. Typically this sticker will have some indication of speed. But since you likely don't know what to look for just type the RAMs model number into google. This model number may contain letters and is typically on a sticker on the RAM.