Heroes that regularly perform above what they should be able to. Usually game defining and very high priority.
Heroes that usually perform well, but can take over a game quite easily when drafted appropriately.
Heroes that should generally be picked as a response to what the opponent has, cannot be taken in a vaccum.
Heroes that require specific conditions to be met in order to be reasonably drafted. This does NOT mean the heroes are necessarily bad, and when they are picked they are usually very dominant. Many heroes in this tier are not commonly understood, so the player usually has better matchup understanding then the opponent.
Heroes that should probably keep to being bench warmers, unless you have a very good reason to play them.