A combination sensorA device that can measure something about its environment. Examples are movement, light, color, moisture, pressure, and many more. with an accelerometerA sensor that measures acceleration and deceleration. and a gyroscopeA sensor that detects when it changes direction., and, often, with a magnetometerA sensor that detects magnetic fields -- from the earth or from anywhere else.. The first two rely on inertiaThe tendency for an object to keep moving (or not moving) at a constant speed if you don’t accelerate or decelerate it. There’s inertia in a straight line and there’s also inertia of rotation – spinning things want to keep rotating in a specific direction unless you force them to change. to tell you where you are and where you’re going.
Inside the Internet of Things - Bryon Moyer Blog