Imagine that two athletes Tom and Harry are running with a uniform speed of 5 m/s on a straight line and circular track respectively. For a layman both Tom and Harry are moving with uniform velocity but for a physicist only Tom is running with uniform velocity because there is no change in his speed and direction of motion. Whereas in the case of Harry who is running along a circular track the direction of motion is changing at every instant as circle is polygon with infinite sides and Harry has to change his direction at every instant.
Thus a body is said to be moving with uniform velocity if it covers equal distances in equal intervals of time in a specified direction.
A body is said to be moving with variable velocity if it covers unequal distances in equal intervals of time and vice-versa in a specified direction or if it changes the direction of motion.
