There are mostly two kind of interface we discuss, when it comes to Operating systems. Initially, when computer were came in the market, there was only one, it was Application Programming Interface which is also referred as API. This definition I got on Google :- "API, an abbreviation of application program interface, is a set of routines, protocols, and tools for building software applications. The API specifies how software components should interact and APIs are used when programming graphical user interface (GUI) components." IF you have used old model windows or unix then you would probably know how frustrating it was to use non-graphical applications. While we use API this help us creating a Graphical interaction with users. So people would know that this a media file and this is a text file. This made computer very easy to use without even knowing how to program a computer or how to give commands. We actually design a layer of upper thing, obvio