AWD vs 4WD: What's the difference

4WD:

Also referred as four-by-four or 4x4, this system is typically used to handle rough and tough situations such as off road or mud/slush.  Basically, it is a system which sends power to all four wheels equally without vectoring(controlling of power between the wheels or axels), which means that all four wheels will spin at same rate respective to the power supplied from the engine.It consists of a device called transfer case which divides power between the front and rear axles. One of the major drawbacks of this system is that it becomes difficult to turn at high speeds as the wheels on the inner side(according to the turn) has less distance to cover than the wheel on the outer side. This leads to the vehicle sliding outwards of the curve. 

In order to tackle this problem, modern 4wd systems are "part-time" i.e., they operate in 2wd mode on normal roads and can be swithced to 4wd when needed. This also increases the fuel efficiency and also decreases the load on the transmission system, which in overall terms, reduces the maintenance and running cost of the vehicle.

Some 4wd vehicles have a transfer case with different settings like 4low or 4 high modes. Where, 4low is used for low speed maneuverability and 4 high can be used for attaining high speeds on slippery surfaces.

ALL WHEEL DRIVE(AWD)

All-wheel-drive is a much more modern and more complex system than 4wd. It is all about varying the amount of power sent to the wheels either mechanically or electronically. It is kind of similar to the part time four wheel drive system.It's also completely automated. It doesn't need any kind of intervention from the driver and hence is much easier to use.They actually detect the spinning of wheels by constantly monitoring the difference in speed between different wheels and reduces power if detects any loss of traction.

The problem here is that due to more complex system, rather than simple mechanical one, these systems are less robust and can't be repaired everywhere. This limits their use to a drastic extent.It also isn't much capable in extreme conditions. AWD system having more parts, increases the overall weight of the vehicle and hence contributes in reducing the fuel efficiency of the vehicle.