Velocity is a vector quantity; that is, it has magnitude (speed) and direction. If a body is travelling at a certain speed and in a certain direction undisturbed it has constant velocity. Thus if a body is travelling at a constant speed in a circle, its velocity is constantly changing.
We know from Newton's first law that a body will retain its velocity unless another force acts upon it. Thus when a body travels in a circle a force must be applied to stop it from travelling in a straight line. This force is the centripetal force, the only force necessary for a circular motion. What is interpreted sometimes as a centrifugal force is the tendency of the object to follow in a straight line, which would bring it outside of its circular trajectory.
Physicists commonly disrecommend speaking of centrifugal forces instead of centripetal forces, because many people draw wrong conclusions from the idea of a centrifugal force. For example, many are not aware that a hammer released by a hammer thrower continues to move in a straight line (in the non-rotating reference frame), tangentially to its previous orbit, and its flight path is only influenced by gravity and friction. Also, the movement of this hammer in the rotating frame of reference, that was used to describe its orbit, could not be explained by centrifugal force. Whereas the model would make one expect a straight movement away from the center of rotation, with radial velocity increasing linearily with time, the hammer would instead, in the rotating reference frame, move spirally, with a radial velocity that converges toward a maximum (its velocity in the associated inert reference frame).