Question: I am currently studying Graph Theory and want to know the difference in between Path , Cycle and Circuit. 

I know the difference between Path and the cycle but What is the Circuit actually mean.

Answer: 
Different books have different terminology in some books a simple path means in which none of the edges are repeated and a circuit is a path which begins and ends at same vertex,and circuit and cycle are same thing in these books.

Books which use the term walk have different definitions of path and circuit,here, walk is defined to be an alternating sequence of vertices and edges of a graph, a trail is used to denote a walk that has no repeated edge here a path is a trail with no repeated vertices, closed walk is walk that starts and ends with same vertex and a circuit is a closed trail.
In a circuit we have can repeated vertices, but we cannot in a cycle.