Edi in non-geek words I would explain like this:
THE THEORY
1) For the computer, any "character" is a *fixed length* combination of bits (0's and 1's). In the old days, the *fixed length* of each combination was "8", and - because of mathematics - you could therefore only have 256 possible characters (2^8=256).
2) Because of the word-wide adoption of computers, in more recent times this *fixed length* has been extended, in order to accomodate characters from different languages, mathematical formulas, common simbols, etc... making possible to have more than 100.000 possibilities (this is the "unicode" mentioned by somebody else in this thread).
3) The fact that your keyboard only has approximatively 102 keys makes impossible for you to access all of the 100.000+ existent unicode characters, but they are still valid characters, as much as a regular "a" or "7", and can be used in any occasion a unicode character is expected by a program.
THE PRACTICE
1) The effect of "flipping" is in fact just an effect: for the computer "ɐ" is not a flipped "a", it is simply a different character, no more special than a "4" or a "]". Humans are the ones who find it "special".
2) There are on the web a lot of utility to produce a "flipped text". One of them (non necessarily the best one) is here: http://swizzy.frih.net/tools/49/flip
Hope what above was non-geek enough! :)
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