You can find 7 minutes of Prototype 2 gameplay here
Prototype 1 was a free-roam game played in New York City where you played as Alex Mercer. You're "gifted" with an experiment that allows you to transform into multiple forms of kickass monsters to fight the military, other monsters, etc. The game is meant to let nothing stand in your way as you rip apart helicopters with your arms.
In Prototype 2, you can cause more chaos by taking the weapons of military vehicles such as tanks and assault jeeps to use for yourself. This game is pure mayhem. It will be released in April 2012.