The reason it's called Legacy is because this packs the entire Star Trek universe into one single game.
You start off ordering around Jonathan Archer in command of the Enterprise, then progress forward in time to the Kirk era, then on to the mighty Jean-Luc 'Earl Grey, hot' Picard period, then move onto Janeway and whatever the bloke was called who was in charge of Deep Space Nine. Never watched that. It was rubbish and nothing like as good as Babylon 5, despite what you may have read on the internet.
It has a gigantic story arc, like sci-fi shows do nowadays (thanks to Babylon 5), one that spans all five of the Star Trek eras. You play the part of an overseeing military commander, a sort of invisible Star Trek god, issuing orders to Archer, Picard and the rest, hence how you're able to follow a story that spans several hundred years. You start out looking after Archer's clunky old Enterprise NX-01, but, as you progress, you're able to upgrade it with alien technology and new Starfleet gear, or you can buy new ships.
You have a mini armada of four Starfleet vessels under your control, and the game starts playing like a weird mixture of 3D exploration and combat. Your ship controls easily - you only really need one thing to control a Galaxy Class starcruiser - the A button. Point the cursor over a planet or bit of space and press A to whiz there.
On Xbox Live you can whiz about as any of the races in the game - humans, Romulans, Klingons or the Borg - with support for eight players at any time. And with each player having an arsenal of four ships, that means you'll be seeing huge space battles consisting of thirty-two ships - all trying to assimilate each other into burning space debris. There will be your standard team game options in here too, like team Deathmatches, territory capture game Planet Conquer, a ship Escort mission and several other of those games sociable people play online where you have to talk to your team-mates.
Better still, all five of the Star Trek captains have provided their voices for the game, so you get to hear the actual Kirk and the rest going through their lines as the battles unfold.