Fallout Tactics is a tactical RPG that can be played in real-time or in turn-based mode. You command a squad of up to 6 Brotherhood of Steel soldiers (Literally, you command them, and are able to issue commands to each one individually) as they go on a campaign across the American midwest. It's prettier and faster-paced than the other Fallout games, but the story isn't quite as strong; the game's empasis is on your guys running around shooting people, not on exploring a post-nuclear landscape.
That's about all you need to know, really. Althout Fallout Tactics is set in the Fallout world, its story doesn't really relate to the other two games' plots. In fact, it's different enough from the other Fallout games that you can't really compare them. It's even been dismissed as "non-canon" because parts of its story don't match up with the other fallout games, but I think it's a decent game unto itself.