It usually consists of recurring characters in a format (usually a 30 minute show once a week) in which there are one or more story lines centred around a common environment, such as a family home or workplace. Traditionally, situation comedies were largely self-contained, in that the characters themselves remained largely static and events in the sitcom resolved themselves by the conclusion of the show. An example of this is the animated situation comedy The Simpsons, where the characteristics of animation has rendered the characters unchanging in appearance forever -- although the characters in the show have sometimes made knowing references to this.
Others, though, use greater or lesser elements of ongoing storylines: Friends, a hugely popular sitcom of the 1990s, contains soap opera elements such as regularly resorting to an end-of-season cliffhanger, and has gradually developed the relationships of the characters.
The situation comedy format originated on radio in the 1920s. The first situation comedy is often said to be Sam and Henry which debuted on the Chicago clear-channel station WGN in 1926. The first network situation comedy was Amos & Andy which debuted on CBS in 1928, and was one of the most popular sitcoms through the 1930s.
Situation comedies have been a part of the landscape of broadcast television since its early days. The first was probably Mary Kay and Johnny, a 15 minute sitcom which debuted on the DuMont network in November of 1947. It was followed by The Goldbergs which first aired on January 17, 1949. Probably the most well-known and successful early television sitcom was I Love Lucy, starring Lucille Ball which is well known because the producer took the step, unusual for its time, of recording the episodes, thereby inventing reruns.
Situation comedies have been produced around the globe. Some countries have developed some distinctive features to the artform - see British sitcom for examples.
A common aspect of family sitcoms is that at some point in their run they introduce an addition to the family in the form of a new baby. However while babies are cute and give adult characters opportunities to act silly, toddlers are of little use in comedy as besides the difficulties of the "terrible twos" they basically can only look cute and say a few words - thus most sitcom kids are aged to four or five within two years of their birth - for example Family Ties "Andrew Keaton" and Growing Pains "Chrissy Seaver". Cases of sticking with the same child such as Erin Murphy's "Tabitha" on Bewitched or the Olsen twins "Michelle Tanner" on Full House are the exception to the rule.
Most contemporary situation comedies are filmed with a multicamera setup in front of a live audience, then edited and broadcast days or weeks later. This practice has not always been universal, however, especially prior to the 1970s when it became more common. Some comedies, such as M*A*S*H, were not filmed before a studio audience. (In the case of M*A*S*H, the use of multiple sets and location filming would have made this impractical).