tl;dr: chef (http://www.getchef.com/chef/) is incredible, let me tell you how it can work for you
It's late at night, and it's a big day tomorrow: your product is going live. All is set up, you've deployed your app and everyone is anxious to start using it. You do a final check before heading home and... disaster. The datacenter where your virtual/dedicated server is being the target of a massive DDoS! They say it'll take hours, possibly days to mitigate. You come to the conclusion you'll have to pull an all-nighter configuring a server in another datacenter.
You start working in a pretty decent computer company at the IT department. Landed a huge job: head of IT! You've been working relentlessly for a week in this awesome disk image to install in all the 70 workstations your developers use. You end up installing everything feeling damn proud of yourself. As soon as the first developer sits down to start working, he utters "you forgot to install the svn client, no one can work". Now you have either to redo the image, or install the svn client 70 times.
It feels quite good to have your own private servers, that take care of your mail, and hosting needs. Oh, wait, one of your servers just became compromised. The one that's configured in a really sneaky way. You'll need a clean re-install.
Your app hosted at amazon needs a couple more dependencies. Time to redo that pesky AMI image. From scratch.
These sound familiar?
None of these need to be a butt-painning, repetitive task that urge you to swear. Attend this talk, and I will show you how chef solves these problems in a clean, lean, clear cut way. You'll only have to configure server functionality _once_ from now on, I promise.
I'll try to explain what chef is, what it does, and how it does it, with illustrative examples, as well as explaining its concepts — such as what recipes, cookbooks, attributes, roles, databags, resources and providers are and how they all get along. I'll explain both modes of operation: with or without chef-server, and what may be the best scenario for you.
Also, should there be enough time, I'll throw in a couple of extras and mention other provisioning software alternatives.