After two failed attempts at professional hockey leagues, and the first world war, the National Hockey League was founded in December 1917. At its inception, the league consisted of five teams; today there are thirty teams actively competing for Lord Stanley's cup each season. You can find more information here: http://www.nhl.com/ice/news.htm?id=381958
Although the game of ice hockey has been around for a very long time, it wasn't until the early 1900s that it became an organized sport including a league. In approximately 1920, Canada formed the NHL and set down the first official set of rules for a league game.
I can tell you what came before the NHL. 1903, the Portage Lakers in Houghton, Michigan organized as a professional team. One year later, the IPHL (International professional hockey league) was formed, of which they were a part as were the Sault Ste. Marie Soos. In 1909, the Canadian Hockey Association was formed and in 1910, the National Hockey Association was formed in Canada, of which the Montreal Canadiens are the only surviving team. The CHA changed some of the rules and hockey went from a 7-player per team game to a 6-player per team game (including the goalies of course). In 1911, the PCHA (Pacific Coast Hockey Association) was formed in Canada.