Two-wheel drive
Two-wheel drive (2WD) describes vehicles in which, in theory, two wheels receive power from the engine at the same time. Usually the two wheels are on the same axle in the drivetrain. Most vehicles with two-wheel drive are mainly for use on roads and highways. Two-wheel drive cars and trucks are either front- (FWD) or Rear-wheel drive (RWD). This means either the front or the rear axle is the "drive axle" that moves the vehicle forward. When the vehicle begins to accelerate, some of the weight of the vehicle is shifted to the rear giving a rear-wheel drive car or better balance and traction. Two-wheel drive systems are simple and rugged. This is why rear-wheel drive is so popular in police cars and other service vehicles.
Disadvantages
Traditional two-wheel drive systems have a differential in the rear axle. This is used for turning the vehicle. If both drive wheels rotated at exactly the same rate, the vehicle would have difficulty going around a corner. In a turn, the outside wheel has to go faster to keep up with the inside wheel (which turns slower). A differential allows one wheel to turn faster under these conditions. This means only one of the two wheels actually has power at any given time. A differential also prevents excessive tire wear.
Of the two drive types, RWD has more disadvantages. In wet or slippery conditions, having effectively only one drive wheel, the tires are going to slip and lose traction. A FWD (not to be confused with 4WD) vehicle will have better traction, but in snow and icy conditions will not equal a four-wheel drive car or truck.
References
Engineering