The SSL (secure socket layer) protocol is the web standard for encrypting communications between users and web sites. Data sent via an SSL connection is protected by encryption, a mechanism that prevents eavesdropping and tampering with any transmitted data. SSL provides businesses and consumers with the confidence that private data sent to a web site, such as credit card numbers, are kept confidential. Web server certificates are required to initialise an SSL session.
The end-user's browser requests a secure channel (via "https:") from the server, and then - if the server has a certificate - the browser and the server negotiate their highest common encryption strength, and then exchange the corresponding encryption keys. The encryption key is then used for this particular instance of SSL, for all from-to exchanges between the browser and the server. The next https session will have a new session key.
The certificate guarantees the security of the connection between the browser and the server.