Why do we need to write test classes and how to identify if a class is a test class in Salesforce?

Testing is a key and critical component to successful long term software development process. Salesforce.com strongly recommends using a test-driven development process which occurs at the same time as code development. Salesforce has very strong set of documentation. When I was learning salesforce unit testing, I realize that it is difficult to understand where to start read. Therefore, I summarized the unit testing for salesforce beginners to understand the basic aspects of unit testing.

There are few things to consider before you deploy or upload the code or package;75% of your Apex code must be covered by unit tests
All the tests must complete successfully
Every trigger has some test coverage (1%)
All classes and triggers must compile successfully
When you are writing a test class, you have to write test for Single Action, Bulk Action, Positive Behavior, Negative Behavior, and Restricted User.
Single Action :Test to verify that a single record produces the correct, expected result.
Bulk Action :  Test not only the single record case, but the bulk cases as well
Positive Behavior :  Verify that the expected behavior occurs through every expected permutation
Negative Behavior :  Verify that the error messages are correctly produced
Restricted User :Test whether a user with restricted access to the sObjects used in     your code sees the expected behavior
Test Class can be defined @isTest annotation. Before the Winter 12’ release we had only private test classes, but on Winter 12’ release salesforce has given the chance to write public test classes as well. Salesforce has released the public test classes for expose common methods for data creation. It can be used to setting up data that the tests need to run against. Public test methods can be called from a running test but not from a non-test request.

When you create a test method,Use static
Use testMethod keyword
Use void return type
No any arguments
No data changes performed in a test method
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Cannot be used to test Web service callout because web services are asynchronous and tests are synchronous.