The Secure Sockets Layer is an encryption implementation used to ensure secure communication between your computer and a remote server, most often used with web sites. You can recognize an SSL connection by the page's URL beginning in "https" instead of "http", and most browsers give a graphical cue to let you you know you're on a secure page, often a padlock icon.

SSL creates its encrypted connection by first verifying the identity of the remote server. By default, browsers trust a handful of companies to issue "certificates", which act like encrypted ID cards. These companies verify the identity of other companies, and then issue them their own certificate. By following this chain of trust, you can be sure that a certificate for Google.com actually belongs to the search company, and not to some hackers who fraudulently applied for a certificate.

Once a connection is established, SSL encrypts the data headed in both directions, keeping it safe from eavesdroppers. This prevents hackers who might be elsewhere on the network from listening in on your traffic. It also stops them from modifying any data while it's in transmission, or sending data pretending to come from you.

A limitation of SSL is that it's only as secure as the computers implementing it. If your PC or the remote web server have already been compromised by hackers, there's little the system can do to prevent further data losses. Despite this, it's still one of the best ways to keep your data safe, especially when using public networks, like those available over WiFi.