The map service is the way that you make maps available to the web using ArcGIS. You'll make the map in ArcMap, then publish the map as a service to your ArcGIS Server site. Internet or intranet users can then use the map service in web applications, ArcGIS Desktop, ArcGIS Online, and other client applications.
What can a map service do?
A map service makes maps, features, and attribute data available inside many types of client applications. One common use of a map service is to show business data on top of basemap tiles from ArcGIS Online, Bing Maps, or Google Maps. Many other ways to use a map service are described in Common reasons for using map services.
How do I start creating a map service?
All map services begin inside ArcMap, where you create your map. When you've finished the map and are ready to share it with others, choose File > Share As > Service > Publish Service from the main menu. The items on the Share as Service dialog box can help you get started sharing your map as a service. See Publish a map service from ArcMap for full instructions.