SSL (Secure Socket Layer) is a server-side service that encrypts data submitted by an online form. It is commonly used in conjunction with shopping carts to secure the transmission of sensitive data, such as credit card information.

When a browser requests an SSL page, usually indicated by a web address beginning "https:" instead of just "http:", it automatically asks the server for a digital Certificate of Authority (CA) to authenticate the server's identity. If there is a problem with the authentication, the browser will usually generate a warning. If authentication is successful, a locked padlock icon will appear in the person's browser, usually on the browser status bar at the bottom right hand side.

Once authentication takes place, the browser viewing the page will automatically encrypt an data sent to it, such as the submission of a form. The type of encryption used is usually 128-bit and is very secure.  The server then decrypts that information once it is received using a secret key. In a similar manner, any information sent by the server over an SSL connection is also encrypted, then decrypted by your browser. 

The whole authentication, encryption and decryption process is seamless and automated - there is nothing the user needs to do.