AI, or Artificial Intelligence are computer scripts/code that can control a civilization in 0 A.D., allowing you to play a game without the need of finding someone to play against.
Mainline AIs[]
qBot[]
qBot is the default AI used 0 A.D. It's the more advanced of the two mainline AIs.
JuBot[]
Originally a marginally improved version of TestBot (a proof of concept AI that performed no actions) JuBot was the first real AI created for 0 A.D. It is no longer actively developed and is kept on for use as the easy AI.
3rd Party AIs[]
Marilyn[]
Marilyn is a fork of qBot developed by wraitii. As of August 2012, Marilyn has been abandoned and it's creator has decided to merge it's improvements into qBot.
RootBot[]
RootBot was a bot created by a group of students for a class project. It is no longer developed, and the last "official" version doesn't work on the latest version of 0 AD. Gudo on the forums has since done one bugfix release here.
Split Bot[]
Created by lexa, Split Bot is no longer under active development and no longer works with the latest versions of 0 A.D. SplitBot code is hosted here on Github.
From the Wildfire Wiki