Force plates measure the ground reaction forces from a body moving over them. They should be able to measure gait, balance and biomechanics specific to their use.
Some only measure load in 2 dimensions whilst the better ones are able to do this in three dimensions.
They contain pedestals, and generally more pedestals are seen as a good characteristic as they measure force pressure over a wider area (covering both heel and toe for example).
Combing a a force plate with video measurements can lead to increased accuracy of measurements, and new measurements such as torque and power to be measured.