Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols which provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, and other data transfers.
There are slight differences between SSL 3.0 and TLS 1.0, but the protocol remains substantially the same. The term "TLS" as used here applies to both protocols unless clarified by context.
The TLS protocol(s) allow client/server applications to communicate in a way designed to prevent eavesdropping, tampering, and message forgery. TLS provides endpoint authentication and communications privacy over the Internet using cryptography.
Typically, only the server is authenticated (i.e., its identity is ensured) while the client remains unauthenticated; this means that the end user (be that a person, or an application such as a web browser), can be sure of whom they are "talking" to.
The next level of security - both ends of the "conversation" being sure of who they are "talking" to - is known as mutual authentication. Mutual authentication requires public key infrastructure (PKI) deployment to clients.
For the rest of the Wikipedia entry on the above term, go here.