Some examples of horizontal circular motion are planetary motion (which may or may not be circular), merry-go-round and motion of electrons around the nucleus.
There is one thing that is common in all these cases. The particle's acceleration is always directed towards the centre of the circle, or in other words, it acts radially inwards.
Where v is speed of the particle and r the radius of the circle
Due to this acceleration, 
we can define a force 
called the centripetal force.
Thus, a centripetal force of magnitude 
is necessary to keep the body in uniform circular motion.
Centripetal force means 'Force towards the centre'. This force must originate from some external force like tension, friction or column force.
There is one more force called the centrifugal force which is a pseudo or fictitious force that is assumed to act only in rotating frames of reference.
