http://g-blog.net/entry/3431
G-Blog.net / Why use [Open]PGP ?
[Open]PGP provides two functions: encryption and, more importantly, authentication. If you get a message from me with a valid [Open]PGP signature attached, you can be almost entirely certain that the message came from me. The message isn’t encrypted; it just has a very small attachment with it. Anyone can read it; those with [Open]PGP [software] can verify it.