Difference b/w Positive and negative test Scenario in Test class in Salesforce?

Positive test Scenario
Test to verify that the expected behaviour occurs through every expected permutation, that is, that the user filled out everything correctly and did not go past the limits.
negative test Scenario
There are likely limits to your applications, such as not being able to add a future date, not being able to specify a negative amount, and so on. You must test for the negative case and verify that the error messages are correctly produced as well as for the positive, within the limits cases.