Smith machine

The Smith machine is an item of equipment used in weight training. It consists of a barbell that is constrained to move only vertically upwards and downwards on steel runners.
The Smith machine was invented by American Jack La Lanne, who rigged up a sliding apparatus in his gym in the 1950s. It was spotted by Rudy Smith, a bodybuilder, who commissioned Paul Martin to improve it. Smith then installed the improved model in a gym he was managing at the time, Vic Tanny's gym in Los Angeles. By the end of the 1950s, Rudy Smith was an executive in Tanny's chain of gyms, and the Smith machine was being manufactured and sold more widely. An article in the July 2005 edition of Muscle & Fitness [1] reported that Rudy Smith was still alive, aged 79, and was the owner of Las Vegas Athletic Clubs.
A Smith machine often includes a weight rack in the base to help stabilise it. Some Smith Machines have the barbell counterbalanced.
Behind each runner is a series of slots on which the barbell can be hooked. Unlike an ordinary barbell, the Smith machine need not be re-racked after a set of repetitions: it can be secured at any point.

Because it cannot fall forwards, backwards or sideways, a Smith machine is considered safer to use than an ordinary barbell. Since the weight does not need to be stabilized, this can allow unstable lifters to lift more weight. There is a risk of force loss by applying it improperly, however, which can reduce the amount of weight lifted.

As the Smith machine constrains the body to a single plane of motion, it does not develop the
stabilization skills, or ability to force the bar into proper form, so lifting ability on it does not translate into freeweight lifting ability, whereas one can always lift on a Smith machine whatever one can freeweight. Weight that requires stabilization, generally freeweights, are thought to be more 'functional', in that they better mimic most physical tasks that the body would normally be asked to accomplish.
Due to the necessity of perfect placement under the bar for proper vertical arc, it is difficult to attain perfect form on the machine, which can result in the force loss, and also result in unusual strains on the body. For those who don't believe some exercises have a perfect vertical lifting line, but rather a slight arc (such as certain bench pressing methods), it is by design impossible to do it this way on the Smith machine.

For these reasons a regular barbell with a power cage may be used for the desired safety.
The Smith machine can be used for any barbell exercise in which the barbell moves vertically up and down, such as the squat or the bench press. It is not suitable for exercises in which the barbell moves in an arc, such as the biceps curl, nor for weightlifting lifts such as the clean and jerk.

http://en.wikipedia.org/wiki/Smith_machine