by providing input values of invalid scenarios.like if the 
user id text box can accept 6-12 characters then a negative 
test case can be sumthing like "enter 5 characters in the 
user id text box and press enter"

test cases written to test that the software is not doing 
what it is not supposed to do

BY GIVING INVALID DATA ,AND TEST THE APPLICATION

Unit Level

Number field        Try to enter other than numbers
Length              Min -1 Max +1
Mandatory fields    Not entering any data and only spaces.
Date Field          If a format is fixed for that field,
                    try to enter other than the specified 
                    format.
Combo Box(drop 
Down Box)           Try to enter some data in the combo box.
Read only Fields    Enter some data in the read only fields.

Try to enter special characters which should not be allowed 
in a field.

Anticipate the response for invalid values , say suppose in
a date field enter a date of invalid for eg.30/02/2007 , the
application should no accept though the date is of specified
format , and shoulg prompt an error message

Based on the Business Rules column in Use Case Document, we 
can prepare the Negative Testcases.

If any queries..

nanda.dreddy@gmail.com

By providing invalid inputs.Negative testing is also called
as 'Test to Fail'.E.g There is 'Name' field (It must contain
characters only).Testing this field by entering numbers is
negative test case in this scenario.