Tier 8 is a type of item set. Each consist of five pieces. Separate 10- and 25-man versions of each set exist, sometimes referred to by players as "Tier 8.10" and "Tier 8.25" (or "Tier 8" and "Tier 8.5") respectively. 10-man sets are all prefixed with "Valorous", while 25-man sets are all prefixed with "Conqueror's". Items from both sub-sets count towards the same set bonuses.
The items that form these sets are purchased with tokens which drop from various bosses in Ulduar. For details, refer to the set page. Note that the bosses drop tokens for different slots depending whether normal (10-man) or heroic (25-man) mode is enabled.