Tier (WoW)  

Tier takes several meanings depending on its context.

1. Talent Trees: When discussing talent trees, tier is often used to refer to the levels of depth in the tree. For example, in the druid Balance tree, Starlight Wrath would be a tier 1 talent since it is available immediately. Control of Nature would be considered a tier 2 talent, being in the row right below it. Brambles would be tier 3, etc. By current BC-era standards, talent trees would go to a depth of 9 tiers. Players often sometimes refer to these by the points required for that level -- "at the 35 point level" rather than "tier 8" -- to be more clear.
2. Raid dungeon class sets: With the rise of The Molten Core and Blackwing Lair class sets, players began creating names to refer to the entire set of equipment out of dungeons. See also: Tier 1, Tier 2, Tier 2.5, Tier 3, Tier 4, Tier 5, Tier 6.
3. Raid progression: Along with the previous explanation, tier has come to also refer to guilds that are working on a current level of equipment. For example, tier 5 set equipment comes from Serpentshrine Cavern and Tempest Keep. Guilds that are working on those two raid zones are usually described as being "tier 5 guilds."