In the Alpha, you'll be able to explore Stratis Island, enjoy a taste of Arma 3's diverse gameplay, and team up with your friends to fight against enemies in 2 co-op mulitplayer scenarios! There are also 8 different vehicles featured in the Alpha and a collection of weapons to help customize your loadout.
Also, if you pre-order the game, you not only get access to the Alpha, you also get some invites to give to friends for the Alpha Lite version of the game, as well as access to the beta and obviously the full game upon release. As an added bonus, all previous Arma titles are on sale on Steam for 50% off until March 12th.