- Accelerometers are basically a mass damping a spring (effectively, not literally). Can be used to detect acceleration in all 3 directions - but using this to estimate position requires a double integral which means there is a lot of noise.
- Gyroscopes measure translational acceleration / rotational speed. Combined with an accelerometer these two can measure all 6 DoF in real space. With a compass, all 3 are the components of an IMU.