Let us take an example:
main() // Here main()---> Calling Function because it is calling calsum
{
int a,b,c,sum; //------> Actual Parameters
printf("Enter values of a, b & c");
scanf("%d %d %d",&a,&b,&c);
sum=calsum(a,b,c);
printf("Sum = %d",sum);
}
calsum(x,y,z) //x,y,z-----> Formal parameters, calsum ---> Called Function
{ // because it is called by some other function
int d;
d=x+y+z;
return(d);
}