What is API Testing?
API is the acronym for Application Programming Interface, which is a software intermediary that allows two applications to talk to each other. API testing is a type of software testing where application programming interfaces (APIs) are tested to determine if they meet expectations for functionality, reliability, performance, and security. API testing is now considered critical for automating testing because APIs now serve as the primary interface to application logic and because GUI are difficult to maintain with the short release cycles and frequent changes commonly used with Agile Software Development and DevOps.
What is UI Testing?
Unit Interface Testing is a software testing type that checks the Graphical User Interface of the Software. The purpose of Graphical User Interface (GUI) Testing is to ensure the functionalities of software application work as per specifications by checking screens and controls like menus, buttons, icons, etc. A user doesn't have any knowledge about a software/Application. It is the UI of the Application which decides that a user is going to use the Application further or not.A normal User first observes the design and looks of the Application/Software and how easy it is for him to understand the UI. If a user is not comfortable with the Interface or find Application complex to understand he is never going to use that Application Again. That's why, GUI is a matter for concern, and proper testing should be carried out in order to make sure that GUI is free of Bugs.