When we check the forum for those inputs which it should 
not supposed to be accept.For example

Negative Testing:
Testing the system in a negative way i.e. Checking that 
wether system is doing what it is not supposed to do.

Negative Testing is simple testing the application beyond
and below of its limits. 

1)We want to enter a name for that negative test can be
first we enter numbers.
2) we enter some ascii characters and we will check
3)First numbers and characters we will check
4) name should have some minimum length below that we will check

Negative testing:

Tests aimed at showing that a component or system does not 
work. 
Negative testing is related to testers' attitude rather 
than a specific test approach or test design technique, 
e.g. testing with invalid input values or exceptions.

Negative test cases for both Edit and cancel button would 
be:
1> It should not accept any key other than mouse click or 
Enter key or a specific combination of key (e.g. Alt-E or 
Alt-C)

Negative Testing is done to show that the software does not 
fail and it remains stable when an unexpected input is 
given.

Negative testing means designing the test cases with the 
sole purpose of breaking the software.

If software does do something that it is not suppose to do 
then it is actually positive testing as software is 
behaving as expected.

Negative test cases for Edit button:Try to enable edit 
button using other hot keys(means if alt+e is the hot key 
then press alt+any other alphabet)

Negative testing means Testing the system using negative
data. eg., for every login page there will be some minimum
and maximum characters to type, so by entering above/below
specified characters we need to check. if it accepts then we
should declare it as a DEFECT(Bug).

for my thinking,nagative test case should be checked min no 
of charactor/max no of charactor and give some special value
(exmpl-123,@,%,_)and try to execute.