Velocity tells you how fast an object is moving in a certain direction. It is calculated as the difference in displacement over the difference in time.
Average Velocity
If the time between the measurements is not instantaneous, then we are calculating average velocity.
Velocity is a vector quantity. As such, it would normally have an arrow above the letter v. However, we indicate that we are taking an average measurement by placing a bar over the top instead.
Instantaneous Velocity
Related: Acceleration, Vector Quantity, Time, Displacement, Kinematics