Negative Testing mean to test the Application with Invalid Inputs. or Enter known data which is not as per valid input still test them.
Example: if any text box limit is 1-10 numeric, then enter into this text box alpha values, special symbols, below 1 & above 10.
Negative test case is writing testcases for the negative teating that is the system must not accept the values.
i think this answer is usefull to you... :)