SSL
From ISSW
SSL (Secure Sockets Layer) provides authentication and data integrity functionality to network connections, most commonly HTTP.
By establishing a framework for peers - whether servers or clients - to use encryption keys, and for those keys to be certified by an certificate authority, SSL allows both for communication to be encrypted, and for the sender to be validated as 'trusted' in some sense.
Most commonly, 'trust' of a peer is ascertained by the existence of a trusted certificate authority, eg. GeoTrust, that has signed the certificate used by that peer. For example, a client browser usually has a set of built-in trusted certificate authorities. A web server wishing to provide secure communication can do so with any key, but in order to be implicitly trusted by the client browser, its key must be signed by one of those built-in trusted certificate authorities. A 'self-signed' key used by the server will (usually) result in a warning message at the client, highlighting that the key being used is not implicitly trusted.
Less commonly, servers can also require that clients be trusted. In exactly the same way, client browsers must have their keys certified by some authority that the server implicitly trusts.
For more information, see the main Wikipedia entry.
