(written from a Production point of view)
Pixar Animation Studios is a computer animation studio, which was founded in 1979 as the Graphics Group, one third of the Computer Division of Lucasfilm Ltd. In 1982, the Graphics Group created the "Project Genesis" demonstration sequence seen in Star Trek II: The Wrath of Khan, and re-used in the subsequent two Star Trek films.
It became an independent company in 1986 with its purchase by former Apple Computers executive Steve Jobs. After aligning with Walt Disney Pictures, Pixar became a prolific producer of computer-animated feature films, with such films as the Toy Story series, Monsters, Inc., The Incredibles (featuring music by Michael Giacchino), Ratatouille (also composed by Giacchino), and WALL-E amongst them. During its years working with Disney, Pixar also provided software support for the production of Star Trek: Insurrection.
In 2006, Disney acquired Pixar but continues to treat it as a separate company.