"Street Fighter II is credited for starting the fighting game boom during the 1990s. Its success led to the production of a series of updated versions, each offering additional features and characters over previous versions, as well as several home versions. Some of the home versions of the Street Fighter II games have sold millions of copies, with the SNES port of the first Street Fighter II being Capcom's best-selling consumer game of all time as of 2008."
Iterations and revisions
Street Fighter II on arcade was ported to several platforms, each of them comprising some adjustments and bug fixes.
Street Fighter II' (aka Champion's Edition) was released on arcade, and with some balance adjustments, also allowed the playing of the four bosses.
Street Fighter II Turbo was released on arcade, and together with some more balance adjustments, also added some extra attacks to several characters. It is generally regarded as one of the best balanced fighter games in the series.
After Street Fighter II Turbo, the next release Super Street Fighter II, which added more characters. See that separate entry.
Source: Wikipedia, "Street_Fighter_II," available under the CC-BY-SA License.