To give an example of what Kelb is talking about:

s'pose your group is composed of a Fighter, a Rogue, a Cleric and a Wizard. If the Cleric is played as a healbot with a mace and the Wizard as a no-frills blaster, there's not much to worry about. With some moderate splatbook usage the Fighter and Rogue can make themselves useful; nothing fancy needed.

The Tiers come into play when the caster players take a closer look at their spell lists and realize what they can do. Both the Cleric and the Wizard can do any job in the party and do it well; basically they become the Angel Summoners while Fighter and Rogue remain the BMX Bandits. Using splatbooks for full casters just adds some bacon, they can break the game just fine using only Core material.

To briefly sum up the meaning of the tiers:
T1: can do any job and do it well, has many potential ways of breaking the game. Such as the "Big Three", Cleric, Druid and Wizard.
T2: has to focus on a more narrow selection of tricks, but in his field of expertise is at least as good as a T1. Has to choose in which ways he wants to break the game. Example: Sorcerer.
T3: Can do his job extremely well, but not gamebreakingly so, and has some other tricks when his primary shtick doesn't apply. Considered by many to be the "sweet spot". Example: Bard (with splatbook usage), ToB classes (Warblade etc.)
T4: Can do his job pretty well, but quickly becomes useless when his primary field of expertise isn't applicable, or does an average job at a slightly wider variety of tasks. Examples: Ranger, Rogue
T5: can do only a single thing, and not necessarily all that well, or is so unfocused that he has trouble doing anything at all. Example: Monk
T6: about the power level of a Commoner.