EzekielRaiden
Follower of the Way
At least in the context of D&D: An idea which came from analyzing 3e.What the heck is "tier stuff" anyway?
In 3e, there were some fairly clear, and meaningful, power levels that different classes could be categorized into. These were useful for explaining how and why clerics were inherently stronger than paladins, for example.
The 3e tiers were as follows:
Tier 1 - Full prepared casters and certain other things (e.g. "spell-to-power" Erudites), mostly Cleric, Druid, Archivist, and Wizard.
Tier 2 - Full spontaneous casters, heavily optimized lesser casters, mostly Sorcerer and Favored Soul.
Tier 3 - Partial casters and the best-designed non-casters. Bard is emblematic of this tier.
Tier 4 - Very limited casters and one-trick-pony non-casters. Paladin and Barbarian are here. Rogues probably go here too, but might rise if strongly optimized.
Tier 5 - Classes that aren't even actually good at their alleged one trick. Fighter and Monk are emblematic here.
Tier 6 - Sometimes given other numbers. This is for things which literally don't work. Like, they literally can't actually do what they're intended to do. AFAIK, the only class in this tier is Truenamer because it's so poorly implemented.
4e didn't really have tiers, as it was extremely well-balanced. It depended more on whether the player made enough reasonably wise choices for character building. So the idea went fallow for 4e's run. Powers and feats and items often got classified, but whole classes really didn't.
5e returned to the 3e model, and consequently brought back the tiers, though they are fuzzier and less dramatic. (5e is better balanced than 3e, but this is like saying that being 1000 dollars in debt with no income is a more balanced budget than being 1,000,000 dollars in debt with no income: true, but not particularly useful.) The 5e tiers, as I understand them, are as follows:
Full casters (Bard, Druid, Cleric, and especially Wizard, but not Sorcerer nor Warlock)
Better partial-casters, strong non-casters, and weaker full casters (Paladin, Barbarian, Warlock, Sorcerer)
Most other non-casters (Rogue, Fighter, Monk)
Known weak/problem classes (Ranger, Artificer)
Individual experiences and personal preference may flex some things up or down, but this is a generally useful structure. Artificer and Ranger stand out as particularly weak classes, and both Sorcerer and Warlock are unusually weak for full casters. Barbarian is arguably weaker than it sounds here, because it's largely being carried by how strong Totem Barbarian is, but it's still decent even without that. Monk is much better than it used to be but is still pretty weak, especially because several of its subclasses are also weak.
Again, I want to note that much of this is at least partially subjective. Not everyone agrees about the precise positions of things on this list. But this is a holistic and overall framework which helps grapple with the comparisons between classes in a given edition.
Hopefully this is a useful answer to your question.