The Unfollow Friend: This “friend” fills your timeline with nonsense but it would be too rude to unfollow him/her.
Uber-cool-for-Twitter friend: You wish this person was a part of your offline life too cos they’re just awesome like that
The DM friend: You basically only DM each other, with no twitter presence whatsoever. Invisibility cloak ftw
The Re-tweet friend: You never utter a word to one another, but diligently re-tweet each others words of wisdom.