A Server is the main computer at your office that provides a central hub for things like your shared files etc. People usually mean file server in this context but there are also mail, web and other servers (not alway within your office though).,
If your office is small you may not even have one.
Computer servers are often made specifically for this task but in smaller offices sometimes a normal computer is used. If this is the case its best that this computer isn't also used by someone doing day-to-day work. Servers are best left as 'dedicated' to their role of servicing the other computers.
Similar there is dedicated 'server' operation system software that is used but again sometime normal computer operating software is used in very small organisations.