This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
PGP is a program that gives your electronic mail something that it
otherwise doesn't have: Privacy. It does this by encrypting your mail
so that nobody but the intended person can read it. When encrypted,
the message looks like a meaningless jumble of random characters. PGP
has proven itself quite capable of resisting even the most
sophisticated forms of analysis aimed at reading the encrypted text.
PGP can also be used to apply a digital signature to a message without
encrypting it. This is normally used in public postings where you
don't want to hide what you are saying, but rather want to allow
others to confirm that the message actually came from you. Once a
digital signature is created, it is impossible for anyone to modify
either the message or the signature without the modification being
detected by PGP.
While PGP is easy to use, it does give you enough rope so that you can
hang yourself. You should become thoroughly familiar with the various
options in PGP before using it to send serious messages. For example,
giving the command "PGP -sat <filename>" will only sign a message, it
will not encrypt it. Even though the output looks like it is
encrypted, it really isn't. Anybody in the world would be able to
recover the original text.