All Wheel Drive.
All wheel drive (AWD) logically must mean that a vehicle has some number of wheels and that all of them are, or can be, driven. Thus four wheel drive is just a special case of AWD where there are four wheels (dual wheels count singly).
Unfortunately some manufacturers are trying to redefine the English language to make AWD or 4WD carry other subtle meanings - usually implying some feature offered on their vehicles, maybe full-time four wheel drive, or 4WD applied to a "car" (e.g.Subaru). The latter is very odd indeed, as all wheel drive is, if anything, the more inclusive term covering six wheel drive, eight wheel drive etc. as found on heavy trucks!
Part Time and Full Time All Wheel Drives:Just as with four wheel drive, one can qualify all wheel drive with part-time or full-time. A part-time all wheel drive delivers power to some wheels (usually 2 out of 4, or 4 out of 6 etc.) during "normal" circumstances, on-road. Drive to all of the remaining wheels can be engaged for slippery surfaces. For example, the ASLAV is a part-time all wheel drive, four wheels being driven normally, all eight in difficult conditions.
A full-time all wheel drive vehicle delivers power to all wheel at all times but requires extra differential(s) to allow front wheels to travel farther than rear wheels on-road or suffer from transmission wind-up. Since the complexity is very great for more than four wheels, full-time AWD is usually confined to light all terrain vehicles (ATV) and a few military vehicles.
4WD Vehicles A-Z
4WD Toys & Models
History of 4WDs