When we want to free a memory allocated to a pointer to an
object then "delete" is used. 
Ex 
int * p;
p=new int;
// now to free the memory 
delete p;
But when we have allocated memory for array of objects like 
int * p= new int(10); //pointer to an array of 10 integer
then to free memory equal to 10 integers 
delete []p;
NOTE: One can free the memory even by "delete p;"  But it
will free only the first element memory.

 

delete is only just to delete a variable which is created 
by new operator. delete[] , it deletes the array where the 
varaialbe points .

If p is pointer to an array and is allocated memory on 
heap, then delete p would call the destructor of the first 
element but will free up the whole block. More info at:

http://www.cppquestions.com/viewtopic.php?f=27&t=13