What is an interface? The best interface is the one of which we do not realize that there is In this context: it is an interface that allows you to interact with a computer system

Usability
The standard ISO / IEC 9126 "Information technology - Software product evaluation - Quality characteristics and guidelines for Their Use" defines usability as "the ability of the software to be understood, learned, used and liked by the user when used under specified conditions" .

The standard ISO 9241-11 "Ergonomic requirements for office work with visual display terminals - Guidance on usability", however, defines usability as "the degree to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use ".

Jakob Nielsen, the "guru" of usability of the web, defines usability as a measure of the quality of the user experience when interacting with something, be it a website or a traditional software application, or any other instrument with which the user can operate. According to Nielsen, a product is usable when it is easy to learn, allows for efficient use, it is easy to remember, few errors allows for interaction and low gravity, it is pleasant to use.
A Brief History of the GUI (Graphics User Interface)

The GUI and Internet

  In 1945, Vannevar Bush wrote:
(...) To future device for individual use, Which is a sort of mechanized private file and library. It (...) is a device in Which an individual stores all His books, records, and communications, and That Which is mechanized so it may be Consulted with exceeding speed and flexibility ".
(ref. www.cs.brown.edu / research / graphics / html / info / timeline.html)
Internet is the realization of that vision utopian and visionary, and we owe it to Tim Berners-Lee, a researcher at CERN, the invention of the World Wide Web in 1990.
(ref. www.cern.ch)

It's with Netscape, the first browser with a true graphical interface, the Web began his advance towards a widespread distribution in the mid 90s.
Since then, the web-design has made great strides as a function of velocity in the connections, the introduction of technologies to overcome the limitations of HTML and the changing needs of boaters.