What is the difference between 2WD, 4WD, AWD?

The difference that the 4wd is selected manually and is commonly used on Pickup trucks. Under normal driving conditions a 4WD vehicle is driven in 2WD(TWO WHEEL DRIVE), meaning power is being transferred from the engine to the transmission, driveshaft, differential and to your rear wheels.

The front wheels of your vehicle are just being pushed along by the rear wheels which are being driven. When you manually hit the 4wd button on your vehicle, it activates a thing called a transfer case. The transfer case allows power to be transferred from the engine to the transmission, through its own driveshaft and to the front wheels. In 4WD the front wheels are now being driven just the same as the rear wheels are.

In an AWD vehicle it operates under the same basic principles as the 4WD system but the AWD system is all four wheels being driven all the time without you having to activate it.