When body of mass $\mathrm{m}$ moves on circular path of radius $\mathrm{R}$ with constant speed $v$, it has centripetal or radial acceleration $a=\frac{v^{2}}{\mathrm{R}}$. Direction of acceleration is toward centre of the circle which is shown in figure.
By Newton's second law of motion necessary force to provide this acceleration $f_{c}=\frac{m v^{2}}{\mathrm{R}}$. Direction of this force is toward centre of the circle hence this force is called centripetal force.
In different situation centripetal force is provided as follows :
$(1) $For planet revolving around the Sun necessary centripetal is provided by gravitational force.
$(2)$ For electron revolving around the nucleus in atom necessary centripetal force is provided by Coulombian force (electric force).
$(3)$ For vehicles moving on level circular track necessary centripetal force is provided by friction force, between tyre and road.