public class RenderTexture : Texture

They can be used to implement image based rendering effects, dynamic shadows, projectors, reflections or surveillance cameras.

One typical usage of render textures is setting them as the "target texture" property of a Camera (Camera.targetTexture), this will make a camera render into a texture instead of rendering to the screen.

Keep in mind that render texture contents can become "lost" on certain events, like loading a new level, system going to a screensaver mode, in and out of fullscreen and so on. When that happens, your existing render textures will become "not yet created" again, you can check for that with RenderTexture.IsCreated function.

As with other "native engine object" types, it is important to pay attention to the lifetime of any render textures and release them when you are finished using them with the RenderTexture.Release function, as they will not be garbage collected like normal managed types.

A render texture only has a data representation on the GPU and you need to use Texture2D.ReadPixels to transfer its contents to CPU memory.

The initial contents of a newly created render texture are undefined. On some platforms and APIs the contents will default to black, but you shouldn't depend on this. You can use LoadStoreActionDebugModeSettings to highlight undefined areas of the display, to help you debug rendering problems in your built application.

Additional Resources: Camera.targetTexture.