What is Input.gyro.attitude exactly?

Could someone clarify what is Input.gyro.attitude exactly? The documentation is very vague.

• Does it include compass direction, i.e. is it always oriented in the same direction or does it drift?

• It seems to loose it's direction if I shake a device, although it gets the position back after a while?

• Is it possible to get more accurate orientation of the device by merging sensors (gyro, compass, accelerometer) yourself? If so how?