A test case is a state under which a tester will decide whether an application/software system is working as it is originally established for it to do. It will decide whether a software program has passed or failed.
It may take many test cases to decide that a software program is considered successful to be released. Test case writing plays an important role and makes a test case an important part of the test execution process.
Read more at http://www.bugraptors.com/writing-effective-test-cases.