The differnce is purly arbitury, and comes entirly down to what the marketing department wants to call it.
There however as many differnt ways of providing power to all 4 wheels as there are manufactors, some are better suited to some situations than others, while many are nothing more than a bad marketing trick and will do nothing more than increase tyre wear and fuel consumption.
However as a very general idea of a very general trend that has occured, generaly vechiles labled as being 4WD will have selectable 2WD/4WD and have an off road bias to thier set up (although most new off roaders are going full time 4WD) while AWD is used on purely road orintated vechiles and are set up more for performance, or safety. Its a term that only really showed up in the 80s, and was used largely by Audi and the Japanese as a way of seperating thier mostly performance 4WD cars from much simpler off road vechiles.