Optical flow is used in most compositing packages from After Effects to Shake, Flame, Nuke, Fusion and toxik.
Its a technology which tracks every pixels in you footage from frame to frame and it records the this movements of pixels in form of vector with analysis and study of this information we will able to track, matchmove and it can be also used have various other uses.
For example It forms the basics of MPEG compression technology and this technology can actually do wonders it had been used in for various complex techniques including Universal Capture in Matrix sequels. Its also much related to Mocap
It future optical flow can be used to track full human motions in our PC using simple compositing software.(http://www.cs.brown.edu/people/black/)