Tier System
Why do you call the Samurai class "Tier 6"? Samurai are awesome!"
First, no they aren't. Second, the Tier System is a ranking of the player character classes for OGL d20 system (D&D 3rd, 3.5rd and Pathfinder), devised by the mix/maxing nerds at Gamesologists Forums. After burning out the bump limit in the forums a few times over, it became clear that some classes are just useless, and some are wickedly broken. These Tiers are for comparing the raw power available to character classes -- a poorly optimized Cleric could be weaksauce next to a Hulking Hurler Fighter. The Tiers aren't about what classes you should *always* pick, but rather making sure the player characters in the party are balanced so that nobody feels like a fifth wheel. With a party of a Druid, Wizard, Bard and Monk, you can tell that the Monk player is gonna have much less fun because she'll be overshadowed by the others.
Some classes can move up and down, either with variants (already mentioned above, or with some serious min/maxing and munchkinism. The original thread says that the "Truenamer" class is pretty broken (as in, not working as intended), and usually ends up at Tier 6 or worse, unless it's min/maxed out the wazoo to be able to spam it's abilities, then it rises to Tier 4.
The following is copypasta, mostly because the forum threads for this keep burning out over at Gamesologists, but partially so that fa/tg/uys can still look down their noses at the math nerds at Brilliant-Gamesologists-dot-Com.
If your game is mostly about the social aspects, diplomacy checks and whatnot, then these tiers have no bearing whatsoever, but then neither do their classes (when negotiating a peace treaty, does it matter how many attacks per round you have?)(when negotiating with humanoids other than orcs, I mean).
Tier 1
Capable of doing absolutely everything, often better than classes that specialize in that thing. Often capable of solving encounters with a single mechanical ability and little thought from the player. Has world changing powers at high levels. These guys, if played with skill, can easily break a campaign and can be very hard to challenge without extreme DM fiat or plenty of house rules, especially if Tier 3s and below are in the party.
- Examples: Wizard, Cleric, Druid, Archivist, Artificer
Tier 2
Has as much raw power as the Tier 1 classes, but can't pull off nearly as many tricks, and while the class itself is capable of anything, no one build can actually do nearly as much as the Tier 1 classes. Still potentially campaign smashers by using the right abilities, but at the same time are more predictable and can't always have the right tool for the job. If the Tier 1 classes are countries with 10,000 nuclear weapons in their arsenal, these guys are countries with 10 nukes. Still dangerous and easily world shattering, but not in quite so many ways. Note that the Tier 2 classes are often less flexible than Tier 3 classes... it's just that their incredible potential power overwhelms their lack in flexibility.
- Examples: Sorcerer, Favored Soul, Psion?, Binder (with the online vestiges)
Tier 3
Capable of doing one thing quite well, while still being useful when that one thing is inappropriate, or capable of doing all things, but not as well as classes that specialize in that area. Occasionally has a mechanical ability that can solve an encounter, but this is relatively rare and easy to deal with. Can be game breaking only with specific intent to do so. Challenging such a character takes some thought from the DM, but isn't too difficult. Will outshine any Tier 5s in the party much of the time.
- Examples: Beguiler, Dread Necromancer, Crusader, Bard, Swordsage, Binder (without the summon monster vestige), Wildshape Variant Ranger, Duskblade, Factotum, Warblade
Tier 4
Capable of doing one thing quite well, but often useless when encounters require other areas of expertise, or capable of doing many things to a reasonable degree of competence without truly shining. Rarely has any abilities that can outright handle an encounter unless that encounter plays directly to the class's main strength. DMs may sometimes need to work to make sure Tier 4s can contribute to an encounter, as their abilities may sometimes leave them useless. Won't outshine anyone except Tier 6s except in specific circumstances that play to their strengths. Cannot compete effectively with Tier 1s that are played well.
- Examples: Rogue, Barbarian, Warlock, Warmage, Scout, Ranger, Hexblade, Adept, Spellthief, Marshal, Fighter (Zhentarium)
Tier 5
Capable of doing only one thing, and not necessarily all that well, or so unfocused that they have trouble mastering anything, and in many types of encounters the character cannot contribute. In some cases, can do one thing very well, but that one thing is very often not needed. Has trouble shining in any encounter unless the encounter matches their strengths. DMs may have to work to avoid the player feeling that their character is worthless unless the entire party is Tier 4 and below. Characters in this tier will often feel like one trick ponies if they do well, or just feel like they have no tricks at all if they build the class poorly.
- Examples: Fighter (could be tier 4 with some shrewd tinkering), Monk, CA Ninja, Healer, Swashbuckler, Rokugan Ninja, Soulknife, Expert, OA Samurai, Paladin (could be tier 4), CW Samurai (with Imperious Command)
Tier 6
Not even capable of shining in their own area of expertise. DMs will need to work hard to make encounters that this sort of character can contribute in with their mechanical abilities. Will often feel worthless unless the character is seriously powergamed beyond belief, and even then won't be terribly impressive. Needs to fight enemies of lower than normal CR. Class is often completely unsynergized or with almost no abilities of merit. Avoid allowing PCs to play these characters.
- Examples: CW Samurai (without Imperious Command), Aristocrat, Warrior, Commoner, Truenamer