In my constant search for information to help understand Amazon Sales Rankings and what these mysterious numbers mean in terms of actual sales, I came across this reasonable and well thought out explanation. The numbers the author proposes on this blog post seem to co-relate to my experience and to be reasonable, although I still believe that only Amazon truly knows what their sales ranking numbers really mean.
The article/blog post can be found at the Nimble Books site at this link: http://www.nimblebooks.com/wordpress/2009/01/understanding-your-amazon-sales-rank/
While this is a reasonable explanation of what Amazon Sales Rankings mean, I still like the service provided by RankTracer which provides you a real number to allow you to track your sales. By a real number I mean that RankTracer takes the feed from Amazon and translates the information into estimated sales for the item you are tracking on Amazon. RankTracer also provides estimates on the number of books sold per day and you can have charts created that show how many books have been sold and the Amazon Rankings over a variety of time periods which can be for a single day, a week or longer. RankTracer is not free but will give you a two month trial and the price per item tracked is very reasonable.