An all-too-quick 40 minutes? At a user/usage level? There's a LOT to choose from, but as a great start, try RFC2504.
Pick and choose as appropriate to your needs. We tried to make it very useful as a reference for the generic user. You can even hand out copies if you like. For a bit more detail, and as a good read in case you get asked some lower-level questions, try RFC 2196, more specifically targeted for IT folks, and "Middle Managers" who have to at least be exposed to the concepts.
PS(don't let the fact that these are TEN years old fool you, most of these concerns are still quite current, most companies (read: those of popular OSes) don't exactly *want* people to understand the why's because they start to question the why-not (yet)s. If you found any of this useful, or not, just reply here, Most if not all those email addresses are defunct at this point -- we've moved onto and into other things).