Unity is the default interface for Ubuntu on PCs and TVs. It is the result of design and development work lead by Canonical and the Ubuntu community. You can participate in Unity design and engineering and the parts of Unity are all published under open source licenses.
Based on the Compiz window manager, Unity has been Ubuntu's default interface since 11.04. An early version of Unity was also used as the interface of Ubuntu 10.10 Netbook Edition. Since Ubuntu 11.10, a 2D version using the Metacity window manager is available as the fallback for computers not powerful enough to run Compiz.
This tag is for questions relating to Unity in general.
If you have a question about a specific part of Unity and aren't sure what it is called, please see What's the right terminology for Unity's UI elements?