Graphical User Interface
The first interfaces were called "command lines". They were just what they sounded like, a line where you could command the computer. You would issue a command in verb-noun syntax, as in copy (verb or action) a file (the noun or object). The problem with the command lines (CLI's), was that to use them effectively, you had to memorize all commands, and remember the proper names and spelling of every file as well -- and type them out quickly and without error. Most users didn't find this a pleasant experience.
In the 1960's researchers were looking for better ways to operate computers. They created concepts like mice and joysticks (so you could point at things on the screen), and windows (so that you could have something contained in one area of the screen and not taking up the whole thing), and menus (so that you could see and organize what commands you could give the computer).
In the 1970's Xerox brought together lots of these researchers and ideas into one place called PARC (Palo Alto Research Center). They did research on creating a new type of interface that would be easier to use. It would be more graphical and use all those concepts that existed before, but work seamlessly and far more easily. What they started was the GUI (Graphical User Interface).
Basically a GUI would use a mouse to select an object (noun), then users could use menus to tell the computer what action to do (verb). The objects (nouns) were visual (little pictures called icons) and not just text names -- so people could pick them instead of having to remember and type out their entire names. And users could issue commands by picking from the list (menus), instead of having to type out the entire command. This alone was much easier -- but many other concepts were added as well.
GUI's used windows to layer documents (so users could work with more than one thing at a time), and instead of using a simple text display for the command line, it was very graphical -- with lots of different character types that looked how they would when printed -- known as WYSIWYG (for What You See Is What You Get). They furthered much of the research that had been started, and brought many ideas together and added quite a few of their own.
But Xerox wasn't creating a product -- they were more a research center. Apple had hired a few people from there, and those people convinced Steve Jobs and others to go back to Xerox and see what they were doing. Apple paid to see what was going on ($1M in stock options), and for the rights to use whatever they saw... and they loved it. They put all these concepts together and created many more of their own for their Lisa and Macintosh computers (released in the early 1980s). These machines forever changed how people operate computers.
A few years after that, Microsoft borrowed (many say stole) the ideas from Apple (and Xerox) and created their Windows Operating System. It was far inferior to Mac -- but it ran on any IBM-compatible PC, while the Mac had its own hardware (Computer). This meant that Macs could run better -- the computer and programs worked much better together -- but Windows meant that people didn't have to buy a new computer to run a GUI, they could run on computers that people already had. Because most people didn't want to buy new computers, and a few other reasons, Windows became far more popular than Macintoshes because anyone could make them.