Statistics is one of the most challenging areas of our product.  While we have automated a number of the common cases so that the average installation does not need to worry about the details of statistics, it is still important for an administrator to have resources in case they are working on a complex installation.

One of our Program Managers, Eric Hanson, wrote a detailed document outlining how statistics work in the SQL Server Query Processor.  I've attached a link below. 

http://www.microsoft.com/technet/prodtechnol/sql/2005/qrystats.mspx