Mike,

Jim has a good explaination of n-tier. In fact you should read Jim's white paper located on his web page! Microsoft also has lots of articles about n-tier theory and development. One which talks about VFP and n-tier is at

http://premium.microsoft.com/isapi/devonly/prodinfo/msdnprod/msdnlib.idc?theURL=/msdn/library/techart/msdn_bko01.htm

I think a key factor in n-tier comes into play with internets and intranets. This is the placement of the middle tier (business rules logic) on a machine other then the user's PC. For example, if your application running on a network in a corporation has 100 people using it at their workstations and you need to change some business logic, you make that change on one computer (the middle tier) and not on the 100 individual user's PCs.

Thanks
Dean