a film that has the look and feel of a television documentary, but with the irreverent humor and slapstick of a comedy, designed to "mock" the documentary or subject it features.
"Drop Dead Gorgeous," which ridicules a local Minnesota beauty pageant, is the illegitimate child of "This Is Spinal Tap" -- a weak link in the people-are-stupid tradition. In that it portrays a group of film students making a documentary about a local witch legend, "The Blair Witch Project" is not unlike those "the making of" movie documentaries with which subscribers to HBO and Showtime are all too familiar.
A mockumentary or mocumentary is a fiction film presented as a documentary film. They are usually comedic, often parodic in nature, and are often presented as historical documentaries with b-roll and talking heads discussing past events or as cinema verite pieces following people as they go through various events.
Movie examples include Spinal Tap, Drop Dead Gorgeous, The Specials (starring Rob Lowe, very underrated). One profound example on TV in Britain that nobody has mentioned is "Brass Eye" - a series of 6 documentaries and one special. They are available on DVD and video and are totally worth a look.