Every week, Exhaust Notes will pick out an industry, technical or obscure term from the automotive world and give you the lowdown on what it means and how it’s used. Already know the answer? Congratulations, you’re ahead of the game.

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This week’s term(s): All-wheel drive versus 4-wheel drive.

Many people assume all-wheel drive (AWD) and 4-wheel drive (4WD) are the same thing. After all, if a vehicle has four wheels, and the vehicle delivers power to all of them, wouldn't they be?

Well, they're similar, but not the same. 4WD (there are myriad variations, but we'll keep it basic for simplicity's sake) means that, as described above, the engine can deliver power to all four wheels. Typically, though, if something is 4WD it is a part-time system, while AWD is a full-time system. More on that in a bit; first, the reason for those systems:

To put it simply, the way your car starts moving (and keeps moving) is a function of both torque (reacquaint yourself here) and traction. But in many ways, traction is the most important: You could have a Corvette ZR1 engine dropped into a hatchback, but if there's no connection between the tires and the ground (i.e., traction), all that power doesn't do a thing. Traction is a function of the vehicle weight and the type of tires you have, as well as a whole bunch of physics stuff. On flat, dry pavement, traction is almost never a problem; on wet pavement, snow, steep inclines, etc., obviously, this is not the case. With a 4WD vehicle, like a rear-wheel-drive Jeep Grand Cherokee, you have the option in low-traction situations to put your car in 4WD -- this, basically, creates an attachment from the front axle to the rear axle, so the engine's torque is split equally between the front and back tires, which increases traction in most cases. In an AWD vehicle, such as the Suburu Impreza, the front and rear tires are always engaged, which makes for better handling, especially in bad weather. 4WD is more typical of SUVs and trucks -- vehicles that might have to tow stuff or move heavy loads -- while AWD is typically marketed for performance (especially brands with rally car heritage, like Subaru) and safety.