ArcGIS Online is a system created by ESRI aimed at providing a common platform to find, share, and organize geographic content and to build GIS applications. The web front end to ArcGIS Online is ArcGIS.com. Through ArcGIS Online you can access maps, applications, and tools published by ESRI and other geographic information systems (GIS) users.
ArcGIS Online provides web services that the user can use as data sources in ESRI products such as ArcMap, and for geoprocessing and analysis.
You can also upload and share your own content through ArcGIS Online. ArcGIS Online enables you to control who can access the resources that you upload. You can create one or more user groups, within which you an share resources and collaborate. ESRI provides services that enable you to host your own content in ArcGIS Online, and you can publish your content as part of a community basemap that users can access freely through ArcGIS Online as a map service. ArcGIS Online also includes tools for creating online maps and for viewing maps created by others.