How did Stanley cup start?

Originally inscribed the Dominion Hockey Challenge Cup, the trophy was donated in 1892 by then Governor General of Canada Lord Stanley of Preston, as an award for Canada's top-ranking amateur Ice Hockey club.

In 1915, the two professional ice hockey organizations, the National Hockey Association (NHA) and the Pacific Coast Hockey Association (PCHA), reached a gentlemen's agreement in which their respective champions would face each other for the Stanley Cup.

After a series of league mergers and folds, it became the de facto championship trophy of the NHL in 1926.

Source: http://en.wikipedia.org/wiki/Stanley_Cup