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Debian Server is a Command line interface and is used for server applications; such as a web server or a NAS (Networks Attached Storage). It has no Graphice Users Interface (GUI) like windows, MAC or Linux Ubuntu (desktop). It is the GUI that slows down all computers so a machine without a GUI runs very fast. The disadvantage is that all communications are via lines of code on a terminal which can be done with a keyboard and screen attached to the Pi or from another computer using a protocol called SSH.
Most people use SSH from their PC of choice and from a Windows machine it is done with a program called PuTTY (a free SSH client - download from here).