Dude, you need to do some searching on your own. But, because I am the nicest guy around, I'll give you the lowdown.
DDR=Double Data Rate. This is the most common RAM technlogy out now, especially for AMD. The most common speed of DDR is PC3200. PC3200 runs at 200MHz, but because it is DDR, it transfers data twice in one clock pulse (the double part). Therefore, it is sometimes referred to as DDR400.
DDR2=Pretty well the same kinda idea as DDR, but with some more pins and typically faster speeds.
DDR=I havent really looked into this very much, as it is not used in regular computers, except in video cards, which is in the form of GDDR3.
For basic questions like this, www.howstuffworks.com
is your friend.