Business Logic Layer ...

Very good introduction about purpose of Business Logic Layer

http://msdn2.microsoft.com/en-us/library/ms978496.aspx
http://www.codeproject.com/gen/design/DudeWheresMyBusinessLogic.asp

And good example of implementation Business Logic Layer in ASP .NET application
(MUST READ):

http://www.asp.net/learn/dataaccess/tutorial02cs.aspx?tabid=63