IGN made the announcement this morning, which includes a few notable details on the game. The sequel features new classes, alien types, and even procedurally generated maps. Also, the trailer shows that the game takes place in 2035 and involves some kind of corrupt future government called Advent.
Perhaps the most important detail is the fact that XCOM 2 is a PC exclusive. The previous game also came out on PlayStation 3 and Xbox 360 and played well on consoles, so it'll be interesting to hear why Firaxis Games shifted to a PC-only release for the sequel.
XCOM 2 is scheduled for a November 2015 release. Firaxis Games plans to reveal more details in the coming weeks, and the first gameplay footage will be shown at E3.