Definition: A table tennis robot is a machine that is capable of simulating some aspects of a table tennis training partner. Robots can be very useful for training technique or working on fitness, when used properly.
Most robots use spinning wheels to eject the ball with various combinations of topspin, backspin and sidespin. In cheaper robots only one motor and throwing wheel is used, and the amount of spin is always in proportion to the amount of speed. More expensive robots have more than one throwing wheel, and are capable of independent spin and speed. The frequency of throwing the ball can also be adjusted on all good robots.
Many robots also use what is called a feed mechanism, where a net is used to catch the balls and recycle them for reuse, allowing many balls to be hit in one session.
Some robots are capable of being programmed to place the balls with different spins and speeds, and to different locations, allowing a more realistic training experience.
There are a number of table tennis robots on the market today, you can read reviews about some of the more popular models here.