Whose Line Is It Anyway?, also known as Whose Line? for short, is a successful Improv slash Sketch Comedy slash Panel Show that originated on British radio before moving to Channel 4 for a ten-year run. Clive Anderson hosted the show, in which four improvisational comics took suggestions from the audience to act out hilarious unscripted scenes. The performers could change radically with each episode, and because of the nature of the show, they could cobble together upwards of four episodes worth of material from one filming session (which also explains how the American version could air as many as 39 episodes each season).
An American version was created for ABC with Drew Carey as host that ran eight seasons (filming actually stopped in 2001 during the show's fourth season, though they filmed so much material that four more seasons aired after filming stopped, the last two of which aired on ABC Family). Because of the wider broadcasting of American television, this show is better known than the British original. In the US, the British version was an early staple of Comedy Central, back in the earlier portion of the 1990s. Ryan Stiles and Colin Mochrie were regular performers on the British version (who between them gave us much Ho Yay) and on this version they appeared in every episode. Ryan and Drew (who worked together on The Drew Carey Show) also provided the show with an ample supply of Foe Yay. From the second season onwards, Wayne Brady was added to the regular line-up, which essentially launched his career. The fourth player varied, the most common performers being Brad Sherwood, Greg Proops, Jeff Davis, Kathy Greenwood or Chip Esten. Throughout the run, Laura Hall and Linda Taylor would provide musical accompaniment, becoming regular players themselves as the show would feature at least one musical game with Wayne (and occasionally Chip, Brad or others), and one Hoedown or Irish Drinking Song.
The shift from mostly rotating players to mostly regulars with an occasional guest - which began in the later British series and became the standard for the American version - made the show's format less 'experimental' and more formulaic, yet at the same time led to the players being more familiar with each others' foibles and led to richer humour. One example of how this changed the show is that in early series the quirks in Party Quirks, the personas in Let's Make a Date and so on were very simple and one-dimensional (e.g. "a sperm"), while in later British series and especially the American version they became comically complex and specific (e.g. "horse whisperer calming and mounting the others who he thinks are wild stallions"), as the players knew each other well enough to discern more details in their performances.
Very much subject to a Broken Base, although the most you'll see these days are a few remarks about how they like or dislike which version or which host.
Despite this, many people on both sides find the two shows hilarious.
Another improv show, Drew Careys Green Screen Show, ran for a season on the old WB channel. A second show featuring the cast and crew of Whose Line Is It Anyway (minus Wayne Brady, but with Brady's Let's Make a Deal sidekick Jonathan Mangum), Drew Carey's Improv-A-Ganza, premiered in Spring 2011 on GSN. And some of the guys (aided by host Fred Willard) got back together in 2012 on ABC with Trust Us with Your Life, a celebrity-based version of the improv game "Day In The Life", where the celebrity tells stories from their life, and the improvisers take it from there. (It is currently on hiatus, rumored cancelled, and currently sits in limbo.)
A revival of the show began airing on The CW on July 16, 2013. Due to Drew Carey's hosting commitments on The Price Is Right, this version is hosted by Aisha Tyler, but apart from that, a revamped set, a few new games, and some new performers, the format is largely the same, with Colin, Ryan, and Wayne as regular performers and a rotating "fourth-seat," which includes some of the familiar fourth-seat performers as well as fresh faces. Laura and Linda also reprise their roles as the show's musicians. The only major difference in the show's format is that the CW version has special celebrity guests on nearly every episode to help out with a couple games and the credits reading. The revival has aired three seasons and began airing its fourth season in May 2016. In October 2016, it was renewed for a fifth season.
As to the show itself, if you don't feel like hunting through cable channels and if you're in either Britain or America respectively, you can watch the UK run legally on 4OD or Hulu (albeit with ad breaks, and Hulu currently only offers the last two seasons). If you're in the US, you can watch every episode of both the first American version and the revival on CW Seed (the CW's comedy division); new episodes are posted the day after they air.
"Welcome to the Whose Line Is It Anyway? series page, where everything's made up, and the tropes don't matter."*
As I tally up the points, it appears that Ryan and Colin are this week's winners... the prize for winning is to read the credits in a style of my choosing. Ryan and Colin, I'd like you to read the credits In the Style of... Solid Snake and Michael Bay at a BBQ party. It remains to me now to thank our regular contributors, you the Internet for reading this, and Richard Vranch on the piano, this is TV Tropes saying goodnight, goodnight.