HBO, short for Home Box Office, is an American premium cable television network, owned by Time Warner.
HBO's programming consists primarily of theatrically released motion pictures and original series, along with made-for-cable movies and documentaries, boxing matches, and occasional stand-up comedy and concert specials.
Since 1977, HBO has produced original programming, which includes dramas and comedies in addition to its slate of theatrical films. Most of these shows are rated TV-MA, and often feature suggestive themes and high amounts of profanity, violence, and nudity - content that would be much more difficult to get on basic cable or over-the-air broadcast channels, for fear of losing sponsors.
Source: Wikipedia, "HBO", available under the CC-BY-SA License.