Circular Motion problems deal with particles moving in horizontal circles with uniform speed and particles moving in vertical circles under gravity.
A body moving at a constant speed in a straight line will continue to move in a straight line in the absence of other forces. If a force, acting in the same direction, is applied to the body, the body will accelerate. If a force, acting in the reverse direction, is applied, the body will decelerate. In other words, forces acting parallel to the line of motion can only change the speed of the body.
In the case of circular motion, a force, acting in a direction perpendicular to that of the body, is applied to the body. The body will change direction and travel in a circle but it will continue to move at the same speed.