In computing an interface is a connector of two, in general, independent entities, these entities can be a computer and a network switch, your browser and a webserver or a GUI and an application.
We separate three general interfaces; hardware, software and user interfaces.
User interfaces are a special kind as they allow a human to interact with an application. These interfaces can be text based like a console, graphical as a GUI or hardware based.
We are only going to look at hardware and software interfaces as these need the most explanation. Some of the interfaces may get their own extended page due to complexity or possible multiple entry points (like with m.2)
These are physical connectors on a device, generally you plug a type of connector into a receptacle.
These are interfaces in application, operating systems and other non physical interfaces. A GUI is a special form of software interface, but normally it is only user or rather human facing in our current world.