In physics, acceleration is the rate at which the velocity of a body changes with time. In general, velocity and acceleration are vector quantities, with magnitude and direction, though in many cases only magnitude is considered (sometimes with negative values for deceleration, treating it as a one dimensional vector). source: Wikipedia
Acceleration is basically the recorded change in the velocity of an object. It is the rate at which the object changes its speed or velocity. Stepping on the gas pedal when driving a car makes it to accelerate.