Disgaea is a Turn Based Tactical RPG much like Final Fantasy Tactics. The game is based in the Netherworld, where the protagonist is has to go out and beat up a bunch of monsters for some reason or another, usually relating in some way to the Overlord of the Netherworld.
I've only played the first game, though. In that one,the main character Prince Laharl has woken up after a two year long nap to find out his father, King Krichevskoy, the Overlord of the Netherworld, is dead. As the rightful heir, he goes out to claim his position to the throne by beating up all of his competition. Along the way he's joined by Etna, his second in command, Flonne, an Angel who was sent to assassinate him but is too nice to do so, and Captain Gordon, Defender of Earth (and his two sidekicks), who's kind of inept overall.
Because the games are made by Nippon Icchi, they are entirely centered around EVERYTHING having stats (some things have stats within stats, etc etc), and the ability to grind them to insanely high levels. You won't actually need to do that for the main story, but you WILL for the massive post game.