SSL works by allowing the client and server to send each other encrypted data that only they can decrypt. SSL negotiates the terms of the encryption in a sequence of events known as the SSL handshake. The handshake involves the following types of authentication:
SSL server authentication requires the server to authenticate itself to the client.
SSL client authentication is optional and requires the client to authenticate itself to the server after the server has authenticated itself to the client.
SSL server authentication requires the server to authenticate itself to the client.