What is the difference in VTOL technology between British Aerospace Harrier II and F-35B?
the F35 has a ton of electronics that the Harrier could only dream about, but I will focus on propulsion differences. 

The biggest is that the F35B uses a separate lift fan in front of the engine, oriented to provide vertical lift that balances the vectored thrust from the rear of the engine. The lift fan is disconnected when flight speed is obtained and covers closed. Roll control at low speed comes from bleed air from the front fan in the engine.



The Harrier uses a much larger version of a bleed system from a single engine to provide both forward located vertical lift/thrust and a similar total diversion system for rearward lift/thrust through four rotating nozzles. All nozzles are working all the time and rotated to a lift/thrust position. A separate bleed system provides roll and pitch control at low speed.