S/MIME and OpenPGP
similarity: both depends on public/private keys to encrypt/authenticate msg ( emails),
difference: how to distribute the public keys.
S/MIME is similar to TLS, need to depends on X.509 certificate
OpenPGP depends on web of trust to distribute the public key.
references:
http://security.stackexchange.com/questions/7874/how-does-pgp-differ-from-s-mime