Titles are aesthetic labels attached to the player's name after completing the game in Diablo II and the Lord of Destruction expansion. You will earn different titles depending on what type of game was completed. These are judged by Difficulty, soft or Hardcore, non-expansion or an Expansion Character, and class gender (in some cases). In Diablo III, titles no longer exist, being replaced with Banners.
Once a title has been awarded, it becomes a permanent feature and can only be changed by earning a new, higher-ranking title.
This is a complete list of all the titles you can earn.