Oh sweet Asmodeus people, the OP is genuinely looking for information. If we want to have our monthly fight about the tier thread can we at least go make a new thread for it?

@Original Poster: Essentially, the tier system measures where classes fall on their ability to affect the game world when all amounts of optimization are equal. So if everyone optimizes at the same level and has the same resources, then Wizard will generally be more versatile and powerful than a Sorcerer, who is in turn more versatile and powerful than a Factotum, who is more versatile and powerful than a Warlock, and so on. It is in no way a value judgement on the 'worth' of a class, and instead measures that class's mechanical ability to solve problems in terms of both breadth and depth.

If players at the table operate at different levels of optimization, then the tier system starts to shake apart. Likewise, at bizzarely low levels of optimization, it starts failing to apply as the players' own inability to comprehend the system takes precedent, but otherwise it's a pretty good way to summarize what to expect from any given class.