The Secure Sockets Layer protects data transferred over http using encryption enabled by a server's SSL Certificate. An SSL Certificate contains a public key and a private key. A public key is used to encrypt information and a private key is used to decipher it.
When a browser points to a secured domain, an SSL handshake authenticates the server and the client and establishes an encryption method and a unique session key. They can begin a secure session that guarantees message privacy and message integrity.