They are servers that provide authentication services to users or other systems. Users and other servers authenticate to such a server, and receive cryptographic tickets.
These tickets are then exchanged with one another to verify identity.
Authentication is used as the basis for authorization (determining whether a privilege will be granted to a particular user or process), privacy (keeping information from becoming known to non-participants), and non-repudiation (not being able to deny having done something that was authorized to be done based on the authentication).
The major authentication algorithms utilized are passwords, Kerberos, and public key encryption.
Read the rest of the article here