WordPress’ popularity has grown exponentially as of late. This rise in popularity is due in part to WordPress’ custom fields. Custom fields allow you to add little bits of data to posts. They have changed the way people look at WordPress. A couple of years ago, WordPress was a blogging platform — a good one, but a blogging platform nonetheless. Now it’s widely considered to be an excellent simple content management system. How did it evolve so quickly? Custom fields, that’s how.
All in all a good read with links out to additional resources.