Developers and testers have different roles, but both are essential to software quality. Developers write code and testers test it. Testers should not be code police, but they are the last line of defense against defects.
There are many different types of software testing (unit, integration, functional, non-functional, etc.), but they all have one goal in common: to find defects.
Unit testing is the most important type of testing because it tests individual software components (units) in isolation. If a unit test fails, it is usually easy to identify the cause of the failure.
Integration testing is done after unit testing. It tests how units work together. If an integration test fails, it can be difficult to identify the cause of the failure because it could be due to a defect in one of the units or in the way the units interact with each other.
Functional testing is done after integration testing. It tests the end-to-end functionality of the software. If a functional test fails, it can be difficult to identify the cause of the failure because it could be due to a defect in any of the software components or in the way the software components interact with each other.
Non-functional testing is done after functional testing. It tests the non-functional aspects of the software such as performance, scalability, security, etc.
Software testing is important because it helps to find defects in the software before the software is released to customers. defects that are found after the software is released can be costly to fix.
It is not always possible to find all defects before the software is released, but the goal of testing is to find as many defects as possible so that the software can be released with confidence.