Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet and in private networks for data transfer. There are slight differences between SSL and TLS, but the protocol remains substantially the same. TLS provides endpoint authentication and communications privacy using cryptography. Typically, only the server is authenticated while the client remains unauthenticated. (Wikipedia)

The SSL protocol runs above TCP/IP and below higher-level protocols such as HTTP. It uses TCP/IP on behalf of the higher-level protocols, and in the process allows both machines to establish an encrypted connection.

See SSL Protocol Overview or Introduction to SSL for detailed description of the protocol.