It is the rate of change of displacement with respect to its surrounding. Since the speed of a body is irrespective of its direction, therefore it is a scalar quantity.
It is also the rate of change of displacement with respect to its surrounding, in a particular direction. Since the velocity is always expressed in a particular direction, therefore it is a vector quantity.
It is the rate of change of velocity of a body. It is said to be positive; when the velocity of a body increases with time and it is negative when the velocity decreases with time.