A Win Forms program can act as a standalone executable or as the client aspect of a two-tier system. Additionally, Win Forms can act as a user-interface layer within a three-tier application. Using HTTP calls to a server that is connected to a database, Win Forms are capable of acting as the front-end to a very powerful, data-enabled architecture that leverages the rich user interface of a client application with the advanced server processing enabled by business logic.
For more information, see Creating a 3-tier Data Application