A little extra info, it stands for Solid State Drive, which means there are no moving parts. It's basically like a large flash drive, but it uses HDD ports instead of USB which are a fair bit faster, so general loading and start-up times can be very quick. It should not replace an HDD all together, though.
SSD's do have less space and shorter lifes so often people have both, (or sometimes it's too expensive or pointless to have just SSD space). The fast SSD for windows and some programs/games, but movies/pictures and other files on the HDD.