XCOM: Enemy Unknown is not a straight remake of UFO: Enemy Unknown / X-COM: UFO Defense, it's a "reimagination". RPS's XCOM review also serves as a comparison/diff to X-COM: http://www.rockpapershotgun.com/2012/10/08/xcom-review/
Ah, the original was called UFO Defense in North America, the recent remake by Firaxis/2k was called Enemy Unknown here. The version in the app store is called Enemy Unknown and appears to be a port of the new version.
http://en.wikipedia.org/wiki/X-COM