Computer icons are a software phenomenon of the late 20th Century. Basically, they consist of small, symbolic pictures which appear on your computer screen and are supposed to help you fathom out what that particular program, function, or button does. They also look incredibly friendly and welcoming when compared to a blank screen and a command prompt. It was the genius programmers at Xerox who developed the first commercial interface using icons in the early 1980s. (Apple like to claim that they invented the icon, but this is patently untrue and typical of the sort of one-upmanship that goes on all the time in the IT industry).
The concept behind icons is that you can indicate what a particular feature of a program does by using an example from the 'real world'. For example, a folder icon to keep files in, or a notepad icon to keep notes in. This is all incredibly subjective, of course. One UK company infamously created a website that used a picture of a Davy lamp* as the 'search' icon. This was the result of having tested the site on local unemployed miners during its development. Of course, the non-mining population of the world remained clueless as to its meaning and the site had to be taken offline after only a few days.
Microsoft, however, adopted the idea for Windows, and of course, suddenly everyone had to have them. This has led, in recent years, to the interpretation of icons becoming stretched in the search for yet more and more obscure symbols to indicate computer functions that have absolutely no real life equivalent (which means just about anything on the Internet). Nonetheless, it appears the humble icon is here to stay.