SSL - A processing layer in your computer that permits relatively secure exchange of data over TCP/IP based networks such as the Internet. The system uses an algorithm to produce a dynamically generated encryption key that is mutually agreed on and then encrypts all data sent over the network with the encryption key.

You will use SSL whenever you perform some form of secure transaction on the world wide web (a credit card purchase for example). You can see when your browser uses SSL because the URL in the address field changes from "http://..." to "https://..." (an 's' is added to the end).