What is GUI Testing?
Graphical User Interface (GUI) testing is the process of testing the system's GUI of the System Under Test. GUI testing involves checking the screens with the controls like menus, buttons, icons, and all types of bars - toolbar, menu bar, dialog boxes and windows, etc.
GUI is what user sees. Say if you visit guru99.com what you will see say home page it is the GUI (graphical user interface) of the site. A user does not see the source code. The interface is visible to the user. Especially the focus is on the design structure, images that they are working properly or not.