What is the difference between "Client/Server" and file sharing?

When using Client/Server there is an active executable program running on another computer. The Client executable program running on your computer talks to this other program through a network connection - making requests for data and other operations. The server program then responds to the client program though the same network connection. A Client/Server setup will allow multiple users to access the same database at the same time. If you simply open a database through Microsoft's networking (drive mapping, etc.) both the client and the server are running locally on your computer. Only the data is on the other computer. This forces the entire database to be "sucked" through the network connection and only one user will be able to access the database at a time.