That's easy. The number of wheels that reciece power. 2wd is either two wheels on the front or back being powered and the other two just rolling. 4wd usually refers to something with the ability to shift between 2 and 4 wheel drive as well as having a transfer case with high and low ranges. AWD is all the wheels getting power but usually only all the wheels, no lock out provision and no low range. 4wd is heavier duty and AWD is usually performance oriented.