I've been meaning to put this in the first post for a while now... I'll link it in later.
A big question that's come up recently is this: what exactly is the purpose of the tiers, and what do they measure? Many people think it means raw power, but that's not the case. Raw power is pretty worthless as a measurement, as there are too many areas to be powerful in. What's stronger after all... the Barbarian that can do 2000 damage a hit, or the Factotum who can be completely impossible to detect? In the end, the best explanation I can give is a metaphor.
Imagine for the moment a map. This map represents the entire campaign world as the DM knows it. The map is four dimensional in that it includes time... it includes the invasion of the Mindflayers 10,000 years before the start of the game, the Orc invasion that's coming a year after the game starts, the fact that the princess of the land will be kidnapped right after the game starts, and so on. This map also includes any notable NPCs, any locations... basically everything the DM knows about (and thus everything that exists in the game world). Some DMs make their entire map before the players even sit down the first time, while others build it piecemeal as the game progresses. Some build a huge map, most of which the players will never see, others build a map that's just big enough to include everything the players will see. But this map exists for all games in some form.
Through this map runs a road. This road represents the path the DM expects the PCs to take through the game world. It too is four dimensional. Perhaps the PCs are expected to be chased out of their starting village by some attacking monsters, then lured to the castle with promises of reward for rescuing the princess, or whatever. In some cases, this road may represent the story the DM wants to tell via the PCs, in other cases the road connects from one encounter to the next. For some DMs, this road will be very thin, and the PCs are expected to follow a very specific set of actions... this is generally called railroading. For other DMs, this road is very wide, such that the PCs can wander all over without ever leaving the expected road... this is called sandbox play. And as with the map, some DMs plan out the road far in advance, while other DMs only plan it out session by session, but in any case there's always some kind of road.
So let's imagine an example, using what we've got above. We've got our campaign world of, let's say, Jaronland. As the DM, I've planned out the continent full of city states, with each city state being controlled by one race and some races controlling multiple such city states. It's a somewhat low magic world... WBL is normal but it's hard to get specific magic items. I'm a pretty sandboxy DM, so the road is pretty wide, but it starts with the PCs getting chased out of their home and sent to a castle where they learn that there is a healthy reward for whoever can rescue the princess, and rescuing the princess will result in the PCs learning of an upcoming orc invasion which they must then prepare for. Eventually they'll gain power and treasure by clearing out some dungeons and solving tasks for allies and amass an army and repel the invasion (okay, so I just finished playing Dragon Age). In this case, the road is that series of events... it's reasonably wide, so as long as the PCs are chasing those goals running around clearing out enemies and making friends they're basically on the road.
Now, what do the Tiers represent? Weaker tier classes will require help to follow my road if I don't specifically build the road to play to their strengths. A Fighter, for example, might be completely worthless when the PCs have to travel over to deal with the elves and convince them to help, as diplomacy is required and the Fighter has absolutely no diplomatic abilities. When dealing with the evil necromancer in his tower, the Ninja can't do anything since all the enemies are undead unless I make sure to include special gear for him. At the very weakest tiers even playing to their strengths won't help... a Warrior will have trouble being useful even in standard combat encounters unless he's heavily optimized. As a DM, I'm going to have to work to make sure my weak tier players can follow my road, by tailoring encounters for them (suddenly, some elves are ambushed by monsters! By defeating the monsters, you make the elves like you more! Good job Fighter!), by giving them loot and gear that fixes their class problems (when you unlock the chest, you find a Truedeath Crystal. Yay ninja, now you can do something useful!), or by otherwise giving them little nudges that help them out (You find a magical warrior only +1 Keen Enfeebling Rapier that's mysteriously Warrior only!).
At the other end of the spectrum are the powerful tier classes. These guys can follow the road easily, but they can also leave it entirely. My campaign as listed would be pretty lame if the Cleric just says "I cast Miracle. There, we win the battle against the Orcs. What's next?" Likewise, having the Wizard assassinate the Orc leader with Love's Pain would be pretty silly. And if the players say "we need to get stronger before the invasion... let's Plane Shift to Ysgard!" things are going to get very weird, as they've gone not just off my road, but right off my map. Suddenly instead of having to help my players along the road, I now have to put barriers on the side of the road to keep them in. This can be nerfs (a mysterious force prevents Plane Shift from working!), coincidences that keep them from using their nastiest tricks (nobody ever loved the Orc leader. Also, he has an antimagic torq that's always on. Stop that), or gentleman's agreements with the players (um, please don't cast miracle in the final battle. It'll mess up my plans. Thanks).
At this point I should mention that I don't consider players to be asses for breaking my game. It's not their fault... really. The rules of the game give them these abilities, and I gave them this scenario, and it makes perfect sense for their characters to do what works in saving their homeland. After all, can you really imagine a Wizard saying "hey, there's the killer dragon that's going to eat us all. I could totally kill it with Shivering Touch and then go home safe and sound with all my friends safe too, but instead I'm just going to cast Haste on the Fighter so he feels better and the fight is more interesting"? That would be like a soldier in battle saying "well, our enemies aren't as well equipped as us, so I'm going to get out of my tank and try and attack them with a sword!" His CO would punch him in the face and get him back in that unfair tank of his right away (or use some other appropriate military discipline). Point being, it's not that my players are asses for doing exactly what they're allowed to do. The problem is the class, not the player (unless the player is being particularly abusive after being asked otherwise, or intentionally messing up the game. But I don't attribute to malice what can be attributed to ignorance). Sure, you can just ask the player not to use the abilities that get the job done really well, but that gets annoying as you quickly run into the situation where the player is saying "okay, I can beat this encounter in the following ways. Which ones are allowed today?" And that's just not a challenging dramatic way to win battles at all.
Anyway, in the center of the Tiers you've got the classes that can follow most roads quite nicely, and yet don't easily go flying right off said roads. These are the Tier 3 and 4 classes. Sometimes they can't follow the road perfectly, sometimes they may be able to leave it, but in general they stay on that road.
And of course you can make particularly easy roads to follow, or hard roads to follow. This works great if everyone's at a similar power level. I'm running a game right now where everyone's a level 6 commoner. I just make the road easy to follow... they most recently had to defeat a group of awakened house cats (the epic battle continues!). In a normal game they'd be screwed, but the difficulty level was set low enough that they could do it (though one of them got sucker punched by a stunning fist to the nuts from a kitten... for one damage. Go Monk Kitty!).
So, one can then catagorize the tiers like this, if one wants:
Tier 6: Can only follow very easy roads. Is virtually incapable of surprising the DM or leaving the road. Will need help to keep up.
Tier 5: Will often have trouble following roads where their specialties don't apply. Will almost never leave the road, though might rarely do something unexpected.
Tier 4: Will occasionally have trouble following roads in certain circumstances. Will very rarely leave the road, but may do unexpected things occasionally.
Tier 3: Will only rarely have trouble following the DM's road. Sometimes will have unexpected abilities that allow them to leave the road.
Tier 2: Will occasionally have trouble following roads in certain circumstances. Will often have abilities that allow them to leave the road, and thus require significant observation to avoid having them go in an unexpected direction.
Tier 1: Will virtually always be able to follow any road not specifically tailored to be difficult for them. Has abilities that allow them to be very unpredictable and can leave even the widest of roads if played with any amount of creativity. Requires significant observation to avoid having them go in a completely unexpected direction.
Note there's a funny thing that happens with the Tier 2 classes, as they're as powerful as Tier 1s (and thus as able to leave the road) and yet they're not as flexible as Tier 3s most of the time. They're just sort of special that way. In a weird way, they're some of the most difficult classes to deal with, as they sometimes need help, and sometimes need restraining. For example, a Sorcerer might be able to Planar Bind something with incredible power to help deal with one situation, and may thus dramatically change your game world (for example by binding a Midguard Dwarf and thus becoming able to get whatever magic items they might want in the example game above), and yet be unable to do something like talk with people and gather information in a town.
Anyway, I hope that all makes sense.
JaronK