The "Completely Automated Public turing Test to tell Computers and Humans Apart",
or CAPTCHA as it's better know, is used as a way to keep "bots" from spamming a guestbook, forum, etc.
It uses randomly generated numbers and letters that are very hard to read with a computer, however they are easily read by a human.
This gives a VERY high probability that anyone submitting the form will be a human and will likely have an actual message.