Have you ever seen an episode of a show that seemed like it could have been a different series? Featuring "old friends" who have never been seen or spoken about before (and likely never again) these episodes could often be removed from the series without anyone noticing or caring. In the world of television, these episodes are referred to as "Back Door Pilots".
Back door pilots are typically created to gauge audience response to a spinoff. The better ones usually involve a pre-existing character who might get his or her own show. The lesser ones introduce previously unseen characters and situations to kick off a show that has little if any connection to the parent show.
An example of a pre-existing character spinoff would be Mrs. Garrett, introduced on Diff'rent Strokes and spunoff to Facts of Life.
An example of the lesser type of spinoff, where an episode introduces a completely unknown character who has never been seen before and never seen again afterwards is Mr. Quiet, an attempt at a spinoff from The Cosby Show starring a Dawn-less Tony Orlando.
The Tony Orlando spinoff would fail miserably, never making it to episode two.
