A TV pilot is a scripted and filmed “first episode” of an intended series. It is usually used to test the waters of whether or not an idea for a TV show is viable and marketable. Often a pilot will never air because no network shows interest in the project. Sometimes one network will produce a pilot, and another network will “pick up” the show and air it, because the original producing network is not interested in it.
There is significant competition for actually getting airtime for a series. Generally a network gives the go-ahead to writers and producers, and will fund production of a TV pilot. Occasionally, a pilot is produced by non-network individuals and is then farmed out to networks to see if interest exists.
If a network expresses interest in the TV pilot, they may use test audiences to see how likely the pilot is to be appreciated, before committing to producing a series. If a test audience responds favorably to the pilot, then the network may choose to make a series, which expands on the premise of the pilot.
Occasionally a TV pilot is kept on the back burner for a while as networks scramble to produce programming that is popular. This can have some interesting results, for example, changes in cast, directors or writers. Most notable are pilots that air and then change something significant, like a cast member, in future episodes. For example, The Waltons first cast Patricia Neal as the mother. Episodes after the pilot cast Michael Learned as Mama.
Sometimes a TV pilot becomes a jumping off point for changing things about a potential series. In the first episode of The Cosby Show, the Huxtables had four children, not five. The oldest child Sondra did not get introduced onto the show until well into the first season.
A TV pilot is usually noted for being much longer than regular series episodes. Some pilots for dramas may be two hours long. A typical comedy pilot may run for an hour. The longer treatment of the subject allows for the network to conclude whether the show may have merit. Networks are not always right, however.
Some TV pilots are instead called backdoor pilots. They can be, for example, a miniseries, that proves so popular it is turned into a series, as in the case of The 4400. Other times a popular show will introduce characters that might later have their own series if reaction to them is favorable.