If an object is moving with changing velocity (either increasing or decreasing), the body is said to be in accelerated motion.
Where, V = Final Velocity, U = Initial Velocity and t = Time taken
Acceleration: In Physics, the rate of change of velocity of an object with respect to time. It is a vector quantity. It's unit is $m/s^2$. To calculated net force is equal to the product of the object's mass and it's acceleration, as described by Newton's Second Law (i.e. $ F$ = $ma$, Where, F = Force, m = Mass and a = Acceleration).
For example: When a car starts from a rest (zero relative velocity) and travels in a straight line at increasing speeds. It is accelerating in the direction of travel called Positive acceleration. If the speed of the car decreases, this is an acceleration in the opposite direction from the direction of the car, it is called the negative acceleration (deceleration).
i) Average Acceleration: Ratio of total change in velocity to the total time interval. This is independent of the velocities at different points of the path but depends only on the velocities at time $t_1$ and $t_2$.
i.e. $a_{av}$ = $\frac {v_2 - v_1}{t_2 - t_1}$
ii) Instantaneous Acceleration: Acceleration is calculated for that particular point of the path of motion at the instant. This gives instantaneous acceleration. And is give by:
Instantaneous Acceleraton ($a$) = $\lim_{\Delta t \rightarrow 0} \frac{\Delta v}{\Delta t}$
This instantaneous acceleration is also called 'acceleration'. It is a vector quantity ($m/s^2$).
Types of Acceleration:
i) Positive Acceleration: Velocity goes on increasing, acceleration is Positive. In +ve acceleration direction of acceleration is in the direction of velocity.
ii) Negative Acceleration: When velocity goes on decreasing, acceleration is negative. It is also called deceleration (retardation). Direction of velocity is opposite to the acceleration direction.
iii) Zero Acceleration: It is also called Uniform velocity. When velocity does not change and remain uniform thus there is no acceleration.