Random Access Memory (often abbreviated as RAM), is a source of memory on a computer's motherboard. It is used to make the computer faster, by saving things to small solid state drives instead of the HDD. This means that the computer constantly read and write page data to the HDD, which would take far longer.
However, Windows includes a feature called "Virtual Memory", where it writes what would go in the RAM to the hard drive, "Virtually" increasing the size of the RAM.
DDR was first used in 1996, until 2000. It's maximum transfer rate was1600MB/s. It was surpassed by DDR2 in 2000, which could transfer at 3200MB/s. In 2005, DDR3 was introduced, and its transfer rate was 6400MB/s. However, DDR3 was not even in enthusist PC's untill 2009, and is still not in wide spread use.