Middle-Tier

It all depand on what application your are intend to write. It can be 2-tier, 3-tier or even multi-tier architecture.

If your application is a 3-tier or multi-tier architecture, then the middle-tier will act as a "Middle Man" which will help the User Logic section to retrieve certain record from the Data Logic section.

Where...
Data Logic = SQL Database.
Middle-tier = Business Logic = Component Object Model (COM). A list of algorithm to retrieve data from SQL database or others.
User Logic = GUI algorithm.

If you application is a 2-tier application then you'll combined the Business Logic and the User Logic into one. Hence, you'll only have:

Data Logic
User Logic

Which may give you a lot of problem when come to the update procudure. Because you need to update each terminal individual as compare to the 3-tier architecture, you only need to update the COM only.
Again, this only happen if you make any changes in your Business Logic.

Hope that what you looking for and what I mention above is correct. :) because I just started my multi-tier application development.