Tal como está escrito em: http://www.cdt.org/crypto/glossary.shtml
Private Key - the secret key of a public-private key cryptography system. This key is used to "sign" outgoing messages, and is used to decrypt incoming messages.
Public Key - the public key of a public-private key cryptography system. This key is used to confirm "signatures" on incoming messages or to encrypt a file or message so that only the holder of the private key can decrypt the file or message.
Public-private Key Cryptography System - a cryptography system that uses two different keys to lock and unlock (encrypt and decrypt) messages and files. The two keys are mathematically linked together. An individual's public key is distributed to other users and is used to encrypt messages to the individual. The individual keeps the private key secret and uses it to decrypt messages sent with the public key.