From The TV IV
A webisode is a short episode (usually 2-5 minutes) of a television series available online to watch either through streaming video or download. Due to increased amount of households with broadband connections, internet-based television has become a more prevalent marketing strategy for popular television series. Because of their length, the webisodes are typically serialized in order to make up one or more larger episodes.
The webisode trend dates back into the late 1990s with WB Animation spin-off series like Gotham Girls and Lobo, as well as a follow-up "season" of The Critic which all took advantage of Flash and other animation technology. During the 2000's, several Adult Swim series, e.g. Perfect Hair Forever, The Brak Show and 12 Oz. Mouse, were granted webisodes.
An pioneer live-action series in this field was Homicide: Life on the Street, which spawn two webisodes in 1997. During the 2000s, due in part to the surge of popularity in video iPods and other digital media devices, mainstream networks finally found valid use in web content, moving prominent live-action series into the forum of internet-only episodes.
Live-action series that, as of the Summer of 2008, have made use of webisodes include Battlestar Galactica (in 2006), Eureka (in 2006), Jericho (in 2006-2007), The Office (in 2006 and 2008), Lost (in 2007-2008, also released as "mobisodes") and Heroes (in 2008).