Essentially Wizards, Clerics, and Druids are at tier 1, the top. They can break the game in a large amount of ways. At Tier 3 are the ToB classes, Bard, and limited caster classes like the Beguiler. This is the balance point most people try to aim for. Tier 5 is the likes of the Fighter and Monk, classes which are just not as good as other classes, especially at what they try to do.
Also applicable to this class is system for prestige class tiers.