Acceleration is the rate of change in velocity of the ground shaking (how much the velocity changes in a unit time), just as it is the rate of change in the velocity of your car when you step on the accelerator or put on the brakes. Velocity is the measurement of the speed of the ground motion. Displacement is the measurement of the actual changing location of the ground due to shaking. All three of the values can be measured continuously during an earthquake.
During an earthquake when the ground is shaking, it experiences acceleration. The peak acceleration is the largest acceleration recorded by a particular station during an earthquake.
Displacement is the difference between the initial position of a reference point and any later position. The amount any point affected by an earthquake has moved from where it was before the earthquake.
Velocity is simplly how fast a point on the ground is shaking as a result of an earthquake.