Not so long ago, in a relative sense, computer graphics were almost an afterthought - the earliest computers didn't have a monitor screen at all. Programmers looked at a wavy line on an oscillocope, a few blinky lights, or a printout, to see if their program worked. Nowadays of course the graphics are the goal, whether it's the latest slick user interface or the most photo realistic game environment. Back in the halcyon days, computer users made the best of what they had; which, initially, was text characters. The ASCII character set is a standard that most computers and printers use to keep track of letters, numbers and symbols - and computer enthusiasts were quick to utilize them as a kind of graphics, when pixels were not available.
In 1966, a man named Kenneth Knowlton at Bell Labs created a photomosaic by scanning a series of photographs - you can see the result above, Studies in Perception I. But just as interesting, and possibly even more creative, are the uses ASCII has been put to by those who manipulate the characters by hand. A great overview is available at the Wikipedia ASCII Art article.
The colon and parenthesis "smiley faces" we still use in our emails today were first proposed back in 1982, and enthusiasts were making good use of text characters in games and simple animation then; the IBM PC initially did not come with a graphics card as standard. Dial-up Bulletin Board Systems - the only way for most people to go online before the Internet became accessible to the general public - were completely text-based for years, and an entire "Art Scene" sprang up around doing the fanciest things possible with whatever color, sound and motion came available. People were trading BASIC programs displaying animated ASCII "Cartoons" in the early 80s, the direct descendant of which you can see in "Star Wars ASCIImation" - or here, with sound added. An entire feature film has also been converted to ASCII.
If ASCII art interests you, be sure to check out Jason Scott's nostalgic collection at textfiles.com (his stated favorite is Angela.art), as well as Joan Stark's site.