If You want to ENCRYPT message: In PKI You use Your private key to decrypt received message. You also share Your public key and well... publicize it to allow other people to create messages which only You can read -message encrypted with You public key, can be decrypted only with Your private key (only You posses this key).
If You want to digitally SIGN message: You use private key to sign a message - everybody can read it (using Your public key), but it is ensured that only message was sent by You.
More info here. [ July 20, 2007: Message edited by: Piotr Uryga ]
Both keys are used for encryption and decryption. In its basic form, works like this: If A sends a message to B, A encrypt the message with A's private key combined with B's public key B decrypts the message with B's private key combined with A's public key
If we limit our discussion about Asymmetric Cryptography there are no public or private keys. It only says that there will be a unique combination of key as 'key A' and 'key B' with which if you encrypt message with 'key A' you will be able to decrypt with 'key B' and if you encrypt message with 'key B' you will be able to decrypt it with 'key A'
Now in real world when you apply this Asymmetric Cryptography in whatever ways *then* you say that, ok.. you designate one of the key as publicly shared and one as secrete/private; only then you will be able to mange secrecy.
So there should not be anything like - only private key can used to encrypt etc. ....make sense?