An Animator Controller allows you to arrange and maintain a set of animations for a character or other animated Game Object.
The controller has references to the animation clips used within it, and manages the various animation states and the transitions between them using a so-called State Machine, which could be thought of as a kind of flow-chart, or a simple program written in a visual programming language within Unity.
The following sections cover the main features that Mecanim provides for controlling and sequencing your animations.
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.