Acouple of releases back, AutoCAD 2010 included a set of parametric drawing tools and, offhand, I can't think of a more significant new feature in recent years. (Don't worry, I'm going to offer a definition of parametric in just a few more paragraphs.) In fact, if you work primarily in 2D, and especially if you're in the manufacturing business, parametrics in AutoCAD (and, to a very limited degree, in AutoCAD LT) might just be the most significant new feature ever.
If you know what I'm talking about, you're probably pretty interested in finding out more about what you can do with parametrics in AutoCAD. If, on the other hand, you think parametrics are the folks who show up when you dial 911, the following paragraphs should straighten you out.