The U.S. Military is expected to purchase over a thousand of the new jets in total, eventually being put in service with the Navy, Air Force, and Marine Corps.
The Air Force version, the F-35A, will be the lightest and most agile. The thrust to weight ratio is over one, meaning that the engine produces more thrust (191 kN!) than the weight of the aircraft. In other words, it is able to speed up while flying 90 degrees to the ground...straight up.
The Marine Corps version, the F-35B, is the most powerful, in that it has a specialized engine. The thrust can be vectored down to "push" the aircraft off the ground, therefore allowing the aircraft to take off in ridiculously short distances (perfect for the Marines' shortened aircraft carriers)
Lastly, the Naval version, the F-35C, has a larger wing area and strengthened landing gear for landing on an aircraft carrier. The wing area is increased simply because this version will have to fly very slow on final, meaning more lift is needed to keep the aircraft from entering an aerodynamic stall. The increased wing area provides more lifting surface area, so (by Bernoulli's principle), more air will flow over the airfoil, inducing a greater low pressure area over the wing. More lift is then created, allowing this model to control itself as very low airspeeds.