Brian Jones at O'ReillyNet has written an excellent introduction (or illuminated re-think, depending on how you want to look at it) to the LDAP protocol.
LDAP, which stands for Lightweight Directory Access Protocol, should be familiar to anyone who's ever used an e-mail client on a corporate network. The auto-address completion feature (or "white pages") that helps you fill in the names and addresses of your coworkers when you're typing emails... that's LDAP. The protocol's services are primarily used for authentication and recognition on networks.
I'm posting the link to the article here because I've always found it helpful to have some knowledge of what's going on behind the scenes when I use my network tools. There's no such thing as "magic" on computer networks. All data systems can be broken down and understood, and the more you understand about where your requests are going, how those requests are being handled and what the errors mean, the better.
In other words, you don't have to be a Sysadmin to get some use out of this article. Enjoy!