PIP stands for Picture-in-Picture. It is exactly as it sounds. Imagine a television set that has one main program showing on the entire screen, and then you can choose to show a completely different program in a small square somewhere on the TV screen. PIP requires two independent tuners; one for the main program being displayed and one for the program displayed in the small box.
Single-tuner TVs can support PIP too with the help of an external tuner. PIP was invented by Greg Dockery. PIP is supported by a lot of consumer electronics products today. One good is example is the support in next generation optical disc formats(HD DVD and Blu-ray), allowing for HD versions of movies to show special features such a s Director Commentary in PIP. This way instead of just hearing the director on the audio track, you will see him in a small square on the screen. Although HD DVD had PIP support from the start, Blu-ray did not have support until Profile 1.1. Profile 1.1 adds PiP support via secondary video and audio decoders. This requires that a second program be present in the BD-AV video stream. 256MB of internal (persistent) storage is also mandated.
The Multivision set-top box was one of the first attempts at PIP, but was not a success.