OpenPGP is the standarized version of
PGP (originally by Phim Zimmermann and now owned by
Network Associates).
OpenPGP defines an infrastructure where public keys can be used to authenticate other parties. It's similar to the PKCS body of standards, but it's not centered around allmighty certificate authorities. Instead of that, in PGP you have a web of trust in which each person can testify for the validity of other people's keys.
OpenPGP is specified in RFC2440. There's a free implementation of this standard called gnupg.