Crusader is a title obtained by earning the achievements  Exalted Argent Champion of the Alliance or the  Exalted Argent Champion of the Horde, which involve earning exalted status with and the right to represent the entire Alliance or Horde in the Argent Tournament, plus exalted status with the Argent Crusade.

The title precedes the character's name in the following format:

Crusader <name>

Keep in mind that if you earn the right to represent every city of your faction, you can no longer do Valiant dailies; however, as of Patch 3.2.0, Crusaders have new daily quests, plus access to new Crusader-only items. In addition, if you use the Faction Change service, this title, and all of the champion's titles that lead up to it (of Stormwind/of Orgrimmar, etc.), will be removed, and you will have to complete the Exalted Argent Champion achievement of your new faction to achieve this title again.