Web Testing could be defined as finding the potential bugs in a web application before making it live or before sending the code to the production environment.
A web testing is important to ensure that your web application is performing effectively on every browser and platforms.
Functional Testing is based on the functionality of the program. It involves only observation of the output for certain input values.
Usability tests helps to find the difficulties faced by people with a product and provide valuable solutions for improvement.
Performance testing is done to find how a system is performing under a particular workload in terms of responsiveness and stability.
Compatibility testing verifies the compatibility of a software on different platforms under different conditions.