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Short for Transport Layer Security, a protocol that guarantees privacy and data integrity between client/server applications communicating over the Internet.
The TLS protocol is made up of two layers:

The TLS Record Protocol -- layered on top of a reliable transport protocol, such as TCP, it ensures that the connection is private by using symmetric data encryption and it ensures that the connection is reliable. The TLS Record Protocol also is used for encapsulation of higher-level protocols, such as the TLS Handshake Protocol.
The TLS Handshake Protocol -- allows authentication between the server and client and the negotiation of an encryption algorithm and cryptographic keys before the application protocol transmits or receives any data.
TLS is application protocol-independent. Higher-level protocols can layer on top of the TLS protocol transparently.

Based on Netscape’s SSL 3.0, TLS supercedes and is an extension of SSL. TLS and SSL are not interoperable.