The term Sitcom is a portmanteau of "Situation Comedy". Originally, sitcoms were broadcast on the radio. Before long, they progressed to television; the Ur-Sitcom on TV is I Love Lucy, which pioneered many of the technical and comedic techniques used to this day, including Three Cameras, but the first television sitcom is generally agreed to have been 1946's Pinwright's Progress on The BBC (all footage is lost, however). The Honeymooners did much to standardize many of these tropes as well.
Generally, situation comedies deal with a small cast of characters encountering humorous situations while living their everyday lives. What counts as "everyday" varies between individuals — daily life for the staff of a bar in Boston is very different from daily life for anachronistic cavemen. Sitcoms are not, however, to be confused with series of other genres that contain Comic Relief.
The two most popular subjects for sitcoms are family life (usually two parents with 2-3 kids) and the workplace full of oddballs. Most sitcoms are a half-hour long (minus commercials), but this isn't a hard and fast rule. Older sitcoms frequently have a Laugh Track. More than any other genre, sitcoms enforce Status Quo Is God.
See also Comedy Tropes.