API TESTING is a software testing type that validates Application Programming Interfaces (APIs). The purpose of API Testing is to check the functionality, reliability, performance, and security of the programming interfaces. In API Testing, instead of using standard user inputs(keyboard) and outputs, you use software to send calls to the API, get output, and note down the system's response. API testing mainly concentrates on the business logic layer of the software architecture and won't concentrate on the look and feel of an application. Whereas In USER INTERFACE TESTING (UI Testing) is used to identify the defects in a product/software by testing using Graphical user interface [GUI]. The purpose of Graphical User Interface (GUI) is to ensure the functionalities of software application work as per specifications by checking screens and controls like menus, buttons, icons, etc.
The GUI elements which can be used for interaction between the user and application:
Simple Example to show the working of UI and API