UI tests check the application via the user interface. In principle, they only have to test whether the user interface works correctly. There are numerous frameworks and tools for testing the user interface. Among those are tools for web UIs and also for desktop and mobile applications. The tests are black box tests. Since they test the user interface, the tests tend to be fragile: changes to the user interface can cause problems even if the logic remains unchanged. Also this type of testing often requires a complete system setup and can be slow to run.
Finally, there are manual tests. They can either minimize the risk of errors in new features or check certain aspects like security, performance, or features that have previously caused quality problems. They should be explorative: They look at problems in certain areas of the applications. Tests that are aimed at detecting whether a certain error shows up again (regression tests) should never be done manually since automated tests find such errors more easily and in a more cost-efficient and reproducible manner. Manual testing should be limited to explorative tests.