Single-user OS is that OS which support only one user at a time. for example: DOS, WINDOWS 3X, WINDOWS 95/97/98 etc.
Multi-user OS is that which support more than one users at the same time. in these types of OS there is a server and all the users are provided different terminals. these terminals are connected to the server. for example: UNIX, LYNIX, WINDOWS VISTA etc.
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Just that- single user has one person using it; multi user many.