From Aquitaine's introduction on the forum:
FreeOrion, an open-source game based on Master of Orion, is a turn-based game of epic space strategy that builds on the classic '4X' model by incorporating the nation-building elements of games such as Europa Universalis 2 and a versatile tactical combat engine. While its modular, open-source design allows for a significant degree of customization of the game engine and the story elements by the community, the FreeOrion team is dedicated to the construction of a living, breathing universe in a 'grand campaign' model.
Or the short version, our summary statement:
FreeOrion brings nation building to a galactic scale with its full-featured grand campaign and in-game racial histories, in addition to the classic 4X model of galactic conquest and tactical combat.
See also the FAQ