A host segment is the name given to the sketches done in-between the movie segments on Mystery Science Theater 3000. They primarily feature the Satellite of Love crew, although the focus can be on characters in Deep 13, Castle Forrester, or other venues. Host segments are usually related to the movie in some way, from featuring a discussion about the film, movie characters dropping in to visit, or simply parodying what was seen in the theater. Occasionally, sketches may have nothing to do with the film at all, making fun of whatever the Brains wanted to at the time.
There were generally six host segments featured in any given episode of MST3K. The first would be the introduction inbetween the opening theme song and the first commercial break. Following the break, the show returns into the first host segment. Each host segment thereafter takes place one quarter, halfway, and three-quarters of the way through the show, with segments two and four before the commercial break, and segment three after. There is then one final host segment that wraps everything up before the show goes to the credits and stinger.
Mystery Science Theater 3000: The Movie has host segments, though there are only four in all, along with an introduction before the opening credits begin. A fifth one (the "storm shelter sequence") was filmed but cut for time.
In the early days of the show, the host segments were written shortly before the show was filmed. Since the writers had not seen the movies featured before and were ad-libbing the theater segments, the early host segments had little (if anything) to do with the featured movie.
When the show moved to national TV and the writing practices changed, several types of segments became fairly common. One of the most enduring was the musical-based sketches featuring the characters singing about things related to the movie. These sketches became prevelent enough that the show was eventually able to create two CD collections of music called Clowns in the Sky.
Later host segments were sometimes connected by a common storyline throughout the show, such as Crow's evil twin visiting, Tom asking Gypsy on a date, Frank getting fired, or the SOL crew entering a mirror universe. The practice became especially common in the Sci-Fi Channel era.
The first segment after the show came back from commercials served as the first part of the show prior to The Crawling Hand. Up until episode 519 Outlaw, the first segment was primarily taken up with the Invention Exchange. After Outlaw, head writer Michael J. Nelson and the rest of the writing staff did away with the Exchanges to free up more space for show-related storylines and sketches.
The final host segment played host to the tradition of reading fan letters, a practice that dated all the way back to the early days of the show. The practice was left behind after the show moved to the Sci-Fi Channel, again to allow more freedom in the show's plots; the last episode to feature a letter segment is Escape 2000. It was paid tribute to in the Sci-Fi-era episode The Deadly Mantis when Dr. Peanut sends his letter letting the SOL crew he escaped the bomb blast that destroyed Deep Ape.