accuracy... Accelerometer is basically something that detects G's in particular directions ie x y or z access, a gyroscope can sense rotation in any direction ... and respond accurate and fast due to the spinning rotor, and gimbal...
its a little more in depth than that but basically think of one of those machines that you attach both your arms and legs to and someone spins you... thats similar to a gyroscope setup except it has a spinning rotor inside of it.. and since its spinning so fast centrifugal force causes it sense any rotation and put out an error signal telling the OS what direction the rotation was...
here's a good article about the differences:
Difference Between Gyroscope and Accelerometer | Difference Between | Gyroscope vs Accelerometer