The checkbox in the top left of the inspector.

There's no label, nor tooltip, for the checkbox in the top-left corner of the inspector when viewing an object. The manual has no single page that describes all parts of the inspector, so I believe I have to more-or-less already know what it does to be able to find it. I have worked out that it makes the object invisible, and makes it ignore collisions, but what else does it do? Where in the manual or reference guide is it?

http://unity3d.com/support/documentation/ScriptReference/GameObject-active.html

I think that's about the best you get. :-|

http://unity3d.com/support/documentation/Manual/GameObjects.html

Deactivating turns off all components. For some, it's the same as disabling the components. However, some components (like colliders) can't be disabled, so deactivating the GameObject may be the appropriate option sometimes. Also, be aware that some functions do not work with inactive game objects.

I think this answer is correct as of now, the other one had a dead link: Unity - Manual: Deactivate GameObjects