What is a test case?
“A test case has components that describes an input, action 
or event and an expected response, to determine if a 
feature of an application is working correctly.” Definition 
by Glossary 

There are levels in which each test case will fall in order 
to avoid duplication efforts.

Level 1: In this level you will write the basic test cases 
from the available specification and user documentation.
Level 2: This is the practical stage in which writing test 
cases depend on actual functional and system flow of the 
application.
Level 3: This is the stage in which you will group some 
test cases and write a test procedure. Test procedure is 
nothing but a group of small test cases maximum of 10.
Level 4: Automation of the project. This will minimize 
human interaction with system and thus QA can focus on 
current updated functionalities to test rather than 
remaining busy with regression testing.

So you can observe a systematic growth from no testable 
item to a Automation suit.

Why we write test cases?

The basic objective of writing test cases is to validate 
the testing coverage of the application. If you are working 
in any CMMi company then you will strictly follow test 
cases standards. So writing test cases brings some sort of 
standardization and minimizes the ad-hoc approach in 
testing.

How to write test cases?
Here is a simple test case format 

Fields in test cases:

Test case id:
Unit to test: What to be verified?
Assumptions:
Test data: Variables and their values
Steps to be executed:
Expected result:
Actual result:
Pass/Fail:
Comments: