This makes the "time shifting" feature (traditionally done by VCR) much more convenient, and also allows for much more "trick modes" such as pausing live TV, instant replay of interesting scenes, and skipping advertising. Most PVR recorders use the MPEG format for encoding analog video signals.
The most popular PVRs on the market are TiVo's Tivo, and SonicBlue's ReplayTV. Many satellite companies are incorporating PVR functions into their set-top box, such as with DirecTivo. In this case, there is no encoding necessary in the PVR, as the satellite signal is already a digtally encoded MPEG stream. The PVR simply stores the digital stream directly to disk.
Upcoming entrants into the market include products from Digeo, Moxi, and Microsoft's Media Center.