Enemy Within is sort of like a standalone expansion for the base game; Enemy Unknown. It introduces new alien enemies, classes, items, missions, and a new major enemy faction in the form of the EXALT complete with its own quest line.
I've gotta say the additions to the base game are substantial and it's hard to go back to the base game afterwards. The rest of the DLC for Enemy Unknown is not very good, but this one is worth getting in my opinion.