Sitcom is a neat abbreviation for situation comedy. It is is a genre of comedy, which features recurring characters in a common environment such as the home or workplace. It is one of the dominant narrative forms on modern television. They are often, but not always recorded in front of studio audiences. Early television sitcoms originated on radio - Hancock's Half Hour being a prime example.
Although the term is situation comedy, it is true to say that the humour is, more often than not, character driven and by its nature running gags often evolve during a series. Often the status quo of the situation is maintained from episode to episode. An episode may feature a disruption to the usual situation and the character interactions, but by the end of the episode, the situation is usually returned to its previous state. There are exceptions to this - some sitcoms do allow their situations and / or characters to change over the course of a series or more.