In simple terms, Accelerometer is a sensor(Hardware) in your Smartphone which is Highly accurate can detect small changes in the position of your phone.
While Orientation sensor & Gravity sensors (Hardware) refers to a less accurate accelerometor sensor can detect only when major change occurs(Ex.Changing the phone from portrait to landscape or vice versa) . As the Orientation sensor & Gravity sensors are less accurate,They cost low. So, they are used in low cost android phone.
Auto-rotation is a software in Android which gets the information from Accelerometer(or Orientation sensor/G-sensor) and changes the your screen orientation.
Note:In phones, Accelerometer can be used for screen rotation,gaming purpose & also to measures the force. But Orientation sensor & Gravity sensors can only used for screen rotation.