3 main interfaces:
1) command prompt
2) menu interface
3) GUI (sometimes pronounced "gewie")
command prompts are things like in ms-dos.. where you have to type things in. old computer style. menu interfaces are things where it says press "1" for this option, "2" for this one etc.
a GUI is basically everything else. windows runs a GUI.
sometimes you'll find something saying "this is program x, and program y is a GUI for x". this means that x is the actual program that does the stuff, but it commonly command line, and so isnt very user-friendly. it still works, but you'd have to type loads of things in. the program "y" would be a GUI, so its just a window with pretty buttons and things you can press. when you press some other button, it'll translate what you rpessed into instructions to pass to the actual program "x". so it really does nothing special, just makes it easy to use.