There are 3 planned versions of the F-35:

Air Force version: the lightest and simpliest one, designed to replace the F-16 and the A-10. It will be dependent on conventional runways. Earlier this year, the Air Force Chief of Staff expressed an interest in buying a few wings of the STOVL version to better provide close air support, a good idea IMO.

Navy version: has more robust landing gear and larger wing area to support carrier landings. This version is designed to replace F/A-18s (A-D models).

Marine/British version: short take off, vertical landing version (STOVL). This is the one with the lift fan. This is designed to replace the Harrier. It's also the most difficult one to build. Many recent articles in Aviation Week and Space Technology magazine have detailed significant weight control issues with the F-35 in general and especially the STOVL version. It looks like they may have to reduce some of the range and payload requirements to meet the weight goals.

When stealth is important, the F-35 will be restricted to internal weapons storage. This will limit its payload to a few bombs and/or missiles. When stealth isn't necessary, it can carry additional weapons and fuel tanks on external pylons. Even then, it won't have nearly the range or payload capacity of a Strike Eagle. For that mission, the Air Force is investigating adding an air-to-ground capability for the F-22. They're also looking at a more optimized bomber version of the F-22 using the maximum number of common systems and designed to carry substancial bomb loads internally for long ranges.