I've seen a few questions mentioning things like "paragon tier" or "epic tier". What are tiers? What game(s) are they used in ? How do they translate to D&D 3.5/Pathfinder Levels?
The tier system was introduced in dnd-4e, and is a more formal development of ideas from earlier editions.
This effectively gives the GM 10 levels notice to plan the character's heroic final fate at level 30, which is where D&D 4e ends.
(The system has developed from a concept present even in very early versions of D&D, that a high-level character would eventually become immortal. The BECMI D&D is the first version with this idea, providing for immortality after level 36. Later editions had the concept of 'Epic levels', beginning at level 21. This progression tended to be slower than at levels 1-20, but to allow otherwise impossible feats, and continue to immortality. In D&D 3rd edition, Epic levels were 21-40, and Deities and Demigods provided limited rules support for becoming gods at levels 41-60.)
The tier system is explicitly used in D&D 4e, but can be applied to most D20 games.
The three tiers of 4e are:
If applied to D&D 3.x it would effectively be
However, epic tier in 3.x is far more different from paragon than in 4e. The rules don't apply perfectly to any system other than 4e (and its derivatives).
Generally, the heroic tier is dealing with threats native to the campaign world, Paragon tier is dealing with extraplanar threats, and epic are some other plane's extraplanar threats... but that's a gross simplification.
D&D 4 Tiers:
The D&D 3 tiers defined in rules
D&D 3 tiers as used before 4E
This breakdown is based strongly in the BXCMI tiers.
The BXCMI Tiers, just for completeness & comparison:
In Basic levels, characters dungeon crawl. While written modules are levels 1-3, the basic sets all include levels 1-5, and basic modules are often not too easy for level 4 and 5 parties.
The D&D3.5 variant E6 gives a 4E-style tier classification for 3.5. Roughly, it's: