Netflix is a U.S. subscription-based online provider of streaming media including both TV and movies.
In October 2011, the CW signed a four year deal to make its shows available on Netflix. Past seasons of Supernatural became available in January 2012. The deal is non-exclusive, allowing the CW to also sign deals with other online services (Source).
Due to licencing issues, changes were made to the music used in season one. For instance, "Carry On Wayward Son" was replaced in the original "The Road So Far" introduction to the episode "Salvation," and "Bad Moon Rising" was dropped in the closing sequence of the season one finale. In season ten, all recaps that play before episodes were edited from the Netflix versions, without explanation from Netflix.