Icons, indices and
symbols are the three types of signs defined by
logician Charles Sanders Peirce. In other words, these terms provide the
basis for a classification of the many ways a thing is considered to "stand
for" or "represent" another thing.
Icon
An icon is a sign that physically resembles what it stands
for. Pictures are icons because they look like the thing they
represent. Onomatopoeic words are icons because they sound like
the object or fact they signify.
Many icons, for example pictures, can be more or less understood
immediately, with no specific training. As for other types of icons, such
as onomatopoaia, resemblance is a vague concept and depends on culture
and habits. See Noises animals make in different languages for
examples.
And yes, the icons on your computer screen are icons, since they usually
look like a printer or a file being opened.
Index
An index is a sign which occurrence implies the
occurrence of some other event or object. The index can be the cause
of the signified thing, or its consequence, or be merely correlated to
it. For example, a smile on your face is an index of your state of
mind. A page on E2 that takes 30 seconds to load is an index that the E2
Web servers are receiving more requests than they can handle
simultaneously.
Understanding indices usually require some thinking or some previous
knowledge. For example, I had to read E2 lag problems and (potential)
solutions to understand what's happening on the Web server.
Symbol
Lazy definition: a symbol is a sign which is neither an
icon or an index.
Real definition: a symbol is a sign which relation to the
signified thing is conventional or arbitrary.
Most words are symbols, because they do not look like or sound like what
they mean (if you believe Ferdinand de Saussure's motto, that linguistic
signs are arbitrary). The colors in a flag are symbols for specific values
(courage, honesty, democracy, union...).
Symbols need to be learned, not only because they are often arbitrary,
but also because their form is very strict. If you change one single letter
in a word, the meaning disappears entirely. If you change the orders of
the colors in your flag, it will not be your flag any more. On the
contrary, you may change details in a painting (icon) or in a smile
(index) with little impact of the signified thing. (1)
Note that other linguists have given very different definitions to this
word. The important point is the idea, not the word.
(1) This is a personal idea, I suppose that other people have
said the same thing (or maybe I'm plain wrong).
Sources:
A good introduction:
http://www.arts.uwa.edu.au/LingWWW/LIN101-2001/NOTES-101/signs.html
Another one: http://www.cs.indiana.edu/~port/teach/103/sign.symbol.html
and others, but you can search Google for the title of this node as I did.