Reasons for Test Cases

Test Cases are mainly step by step procedure for checking the functionality of the application.
If a each module functionality has to be checked Test cases has to be written from looking into requirements
From this at the initial stage itself the issues or specially known as Bugs can be found out and it would be easier for the developer to fix the issues rather than at the end point where everything has to meet the deadline of the Project.
And the Tester also gets to know the Flow of the application where all possible inputs can be tested using Functionality testing
Test Case structure:
Test CaseId:
Description    
Expected Result:
Actual Result:
Pass/Fail
Difference between Positive Test case and Negative Test Case
Many of them get confused on what are Positive Test case and Negative Test case

Well to make it simpler the,
Positive Test case are Valid Test Cases u enter
Negative Test case are In Valid Test Cases u enter.
Most of the Testing has to be done using Negative Test case as the main objective of Testing
is to Find Bugs and that is by entering Negative scenarios that will try to break the application
and make it stable.
Testing is mainly concept of Destructive approach than Constructive approach