These are the computers that you sit in front of to do your work. They will be connected to your 'server' through your network. They need to be fully functional computers in their own right.
These need to run the Windows operating system; anything from Windows XP and after is suitable, e.g. XP, Vista, 7 etc. Windows 95/98/Me/NT/2000 are not supported.
They do not need anything other than Windows itself. Any PC you bought recently will be more than adequate.