Testing is the process of evaluating a system or its components to identify differences between expected and actual results. It involves executing a system to identify gaps, errors, or missing requirements. There are different types of testing such as unit testing, integration testing, system testing, and acceptance testing. Testing can be manual, automated, functional, or non-functional. Functional testing evaluates system functionality while non-functional testing evaluates attributes like performance and security. Common non-functional test types are load testing and stress testing which analyze a system's behavior under different load conditions.