The Tier List is a classification system that describes the relative success rates of characters in most fighting games, when played by computer or high-level players competitively. The Street Fighter series is no exception, and there are various tier lists for most games in the series.
It should be noted that tiers only effect two people facing each other of equal skill. Tier lists tend to change over time when new strategies and such are found for fighters, so there is truly no "final" version of a tier list. There have been many massive debates regarding the legitimacy of tiers, and if they truly even exist in Street Fighter.