I'm not sure what to say about this beyond just regurgitating the Publisher's Description. If that's not enough to make you want to pick this up for at least a glance, I don't think I could say anything that will.
But, quickly, the idea behind the book is that using drawings/diagrams and the 1,000 most commonly used words to explain how various and sundry things work -- from ball-point pens to the Large Hadron Collider, and many points in between and beyond.
The artwork is like what you see on xkcd, the humor is similar -- more broadly appealing (I think), however. The material is educational. That's about it in a nutshell. I guess I should state that the artwork isn't like the stick figure comics, it's the incredibly detailed diagrams that show up at other times.
To give you a taste, some of the entries I found most educational/interesting/amusing were: Shape Checker (padlock); Bags of Stuff Inside You (internal organs); Hand Computer (smart phone); Stuff You Touch to Fly a Sky Boat (airplane cockpit). From Cover to Cover (literally) Munroe's drawings will be teaching you a little more about how things work.
This is not a sit down and read cover-to-cover kind of book, period. It's a grab it every now and then, flip through until you get to a page or two (or six) that you haven't read, or one you have and want to re-read. I'm just leaving this around for my kids (and myself) to pick up from time to time. I wish I could've got a picture of the expression on my youngest's face just now as he was reading through the book, I could've not said a word and simply posted it -- I'm not sure how much he understood, but he was fascinated. I'd gladly buy a sequel -- actually, I could probably use one.