For each serious software project out there, it is highly recommended to have plenty of tests in place. There are various test categories such as unit tests, integration tests, stress and load tests, and end-to-end tests. I have tried to visualize the different categories in the following screenshot:
Unit tests assert the correctness and quality of an individual, isolated piece of the overall application or application service. Integration tests make sure that pieces that are closely related work together as expected. Stress and load tests often take the application or service as a whole and assert a correct behavior under various edge cases such as high load through multiple concurrent requests handled by the service, or by flooding the service with a huge amount of data. Finally, end-to-end tests simulate a real user working...