BCGEIB - I still work for Lockheed, until the good Lord decides to put me in a Marine uniform and slap some wings of gold on for extra measure.
The X-35 is going to be an amazing aircraft. Instead of the three (or four - I can't remember) glass screens on the F-16 - the F-35 will have one large screen that the pilot can use to toggle info he needs to see. There will be a helmet mounted cueing display and what is called Prognostics Health Mgmt. - if a part on the aircraft is about to fail - the computer systme automatically notes that in a database, sends the information to the airbase, so when the plane lands, another part is on its way - all automatic.
TNWhisky - to answer your question - it does have three variants like bcgeib described. The A model is the CTOL (Conventional Takeoff and Landing) aircraft and is designed for the Air Farce. The B model is the STOVL variant (Short Take Off/Vertical Landing) and will be used in the Marine Corps and the Royal Air Force (U.K.) The C model is the Navy variant (CV - Carrier Version/Variant).
All three variants maintain over 80% commonality of parts and airframe. The Navy version has beefed up landing gear and a larger wing area (wing desing is the same - they just have extra portions at the outer edges) The STOVL and CTOL use the same airframe - where the lift van in the STOVL is, the CTOL and CV versions have an extra fuel tank.
Most of the systems are a drastic improvement over the F-16. I don't know about it being underpowered - it uses the same engine as the F-22 - except it only has one instead of two. I guess when I get to fly it, I'll let everyone know.... I think I remember it having around 30 - 35,000 lbs of thrust - don't know how that compares to the latest F-16. One thing, though - is that internal fuel stores is a drastic improvement over the F-16.
I'll go off on a sidenote to follow jarhead's comments. The A-10 is awesome - wish the Corps used that for CAS.