Pronation is, the way that your foot contacts the ground as you are running, and how it pulls or pushes you along to keep you moving forward. Essentially it is whether you use the outside of your foot (as I tend to), the inside of your foot, or are somewhere in between.
None of these habits are inherently bad, but they do make a lot of difference in the shoes that should be worn. I could write an entire article on this topic alone, but the folks ad Runner's World have already taken care of that for me, and to be honest they are certainly more qualified. Take a look at the link below.
http://www.runnersworld.com/article/0,7120,s6-240-319-327-7727-0,00.html
