This is the classical Turing Test:
Two humans involved, directly.
With Captchas (or Funtchas for that matter - let's say "Automated Turing Tests"), it look more like this:
Directly involved is only one human: The poor user. All the other guys are robots: spambot and antispambot. Or computers, or programs, for that matter. Then, we have more humans, indirectly involved: the actual spammer, controlling the spambot by means of programming and the antispammer, controlling the antispambot by the same means.
Poor user, all alone with the bots...