The Animator Controller Asset

When you have an animation clipsAnimation data that can be used for animated characters or simple animations. It is a simple “unit” piece of motion, such as (one specific instance of) “Idle”, “Walk” or “Run”. More info
See in Glossary ready to use, you need to use an Animator ControllerControls animation through Animation Layers with Animation State Machines and Animation Blend Trees, controlled by Animation Parameters. The same Animator Controller can be referenced by multiple models with Animator components. More info
See in Glossary to bring them together. An Animator Controller asset is created within Unity and allows you to maintain a set of animations for a character or object.

Animator Controller assets are created from the Assets menu, or from the Create menu in the Project windowA window that shows the contents of your Assets folder (Project tab) More info
See in Glossary.

In most situations, it is normal to have multiple animations and switch between them when certain game conditions occur. For example, you could switch from a walk animation to a jump whenever the spacebar is pressed. However even if you just have a single animation clip you still need to place it into an animator controller to use it on a Game Object.

The controller manages the various animation states and the transitions between them using a so-called State MachineThe set of states in an Animator Controller that a character or animated GameObject can be in, along with a set of transitions between those states and a variable to remember the current state. The states available will depend on the type of gameplay, but typical states include things like idling, walking, running and jumping. More info
See in Glossary, which could be thought of as a kind of flow-chart, or a simple program written in a visual programming language within Unity. More information about state machines can be found here. The structure of the Animator Controller can be created, viewed and modified in the Animator WindowThe window where the Animator Controller is visualized and edited. More info
See in Glossary.

The animator controller is finally applied to an object by attaching an Animator component that references them. See the reference manual pages about the Animator component and Animator Controller for further details about their use.