Gavin's Sitcom
From The Extended Group
"Gavin's Sitcom" is a description of the group from the perspective of Gavin that is similar to how it would be if our lives resembled that of a sitcom. This includes refering to years as "seasons", labeling people that we only know from one class as being "extras", placing the label of "secondary star" to people that are of only perpherial importance to the group, and "stars" to those who the show revolves around. Because this idea is the world as seen from Gavin's eyes, places such as UW classrooms and dorm rooms that Gavin hasn't actually been are spoken of as if they don't really exist but are only mentioned by characters off screen.
In this way places that were rarely visited by the group, such as Jamie's basement, were mentioned as having been destroyed by an unseen disaster during filming so that they had to be written out of the show. In this same way the "Kevin's House" set was said to be a standing wall with a door that Kevin would walk out of and into a car, with the tape recording of a dog barking playing inside. Characters that were mentioned but never actually seen by Gavin, such as Rama's extended family and Jordan's brother, were explained away as not having been cast yet. After significant events such as when Ganesh and John Chang got in a car wreck it was speculated that this was intended to be a way to write the unpopular "John Chang" character out of the show and elevate fan-favorite Ganesh to regular status. However, it is believed that the producers chickened out and merely put him in the Marines where he would never be seen or heard from again.
Due to the similarity of some characters to one another, characters such as Kevin's brother were played by Kevin with darker hair, and the character of Mitchell was only necessary when the group actually went to Stuart's house and in all phone conversations his voice could simply be done by Stuart.
Guest Stars
Popular guest stars have included Pietro, Stuart's dad, Andy Dustan until he left the show following a violent contract dispute and a bizzare story arc, Elias, and James Zetlen.
Origin
This was first explained in detail when the group was hanging out at Lukas' and eating burgers.