What is test case in Software Testing.

Test case is set of conditions, steps , variables using which tester need to test application or software in step by step manner to find out , verify , validate software is as per requirements and detect defects. 
There may be many set of test cases to fulfill requirements .Main characteristics of a test case is that there is a known input  and an expected output (Expected Result ), which is worked out before the test. The known input should test a pre-condition and the expected output should test a post-condition ( Actual Result).