I don't think most people here have it right. From my understanding the main difference is that 4WD can be manually turned off (meaning going to 2WD mode). AWD is always on.
Some AWD systems might intelligently change power to different wheels (or front and back) and that might vary per brand and model. 4WD could also do this. Usually with 4WD you might also have a 4 lo and 4 hi gear as well.
Then there's also various types of differentials and locks and whatnot which is a whole more detailed discussion.
The RAV4 dynamically engages the back wheels with no mechanical coupling to the front wheels. It uses a separate electric motor in the back. Toyota calls it AWD.