Define the basic difference between API testing and UI level testing?

UI testing means the testing of the graphical interface. Its focus is basically on the feel and looks of an application. Within user interface testing, things like how the user interacts with app elements such as images, fonts, layouts, etc are checked.

On the other hand API, testing allows communicating between two different software systems. During this testing, a software system that implements an API includes sub-routines or functions that can be performed by other software systems.