What is Testing , Why testing require and Who do the testing ?

What is Testing: 

Testing is to show that system is doing what it supposed to do and its not doing what it should not supposed to do. Testing is to show that there are defect in the system but cant prove that there is no defect.

Why testing require:

Testing required to prove the quality of the product by finding the defect. Once defects fixed we can say that quality can be improved by testing.

Who do the testing :

At different stages , different roles do the testing . As at unit level developer do the testing, integration level and system level testing done by Tester, User acceptance testing is done by client or end user.

What are the different types of testing:

Desktop application testing , web based application testing,  database testing, client server testing