SSL (Secure Sockets Layer) is a protocol designed by Netscape Communications to enable encrypted, authenticated communications across the Internet. SSL is used mostly (but not exclusively) in communications between web browsers and web servers. URL's that begin with 'https' indicate that an SSL connection will be used.
SSL has 3 important features:
In an SSL connection each side of the connection must have a Security Certificate, which each side's software sends to the other. Each side then encrypts what it sends using information from both its own and the other side's Certificate, ensuring that only the intended recipient can de-crypt it, and that the other side can be sure the data came from the place it claims to have come from, and that the message has not been tampered with.