Negative testing is a technique that validates that the system behaves gracefully in case of any invalid inputs. For example, in case the user enters any invalid data in a text box, the system should display a proper message instead of the technical message which the user does not understand.
Negative testing is different from positive testing in a way that positive testing validates that our system works as expected and compares the test results with the expected results.
Most of the time scenarios for negative testing are not mentioned in the functional requirement documents. As a QA we have to identify the negative scenarios and should have provisions to test those.