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LDAP (Lightweight Directory Access Protocol) is basically a way of organizing information and providing access to it. It's commonly used for user, service and machine information, and it's incredibly useful. The Linux version is OpenLDAP. It will handle authentication as well as information (so the password aspect of login as well as looking up who the user is, where his home directory is, and so on). However, it's not as secure as it could be by default connections are unencrypted, so your password is being sent out in the open.
Previously, you could deal with this using SSL tunneling, but that has been deprecated since the retirement of LDAPv2 (in 2003). TLS is another option, but even using that, LDAP still has security problems.
For security, your best option is Kerberos. Kerberos is specifically designed to handle authentication. It will not do the information lookup that LDAP does. It avoids password security problems and enables single-sign-on (a very useful feature!).
This article was first published on ServerWatch.com.