See ESRI's Learn ArcGIS Online tutorial.
Visit MassGIS' ArcGIS Online home page. Note: an ArcGIS Online account is required to access any ArcGIS Online content. ESRI ArcGIS Online is a new service which offers many functions. Some of them are listed below, with MassGIS' involvement:
1) Catalog of services
ArcGIS Online allows organizations to post links to ArcGIS Server mapservices, WMS services, applications or documents. Extensive use of tags or keywords is required which enhances discoverability of content for everyone. MassGIS has listed the 4 mapservices from tilesets that make up our Massachusetts basemap. Also listed are other mapservice tilesets such as Digital Globe imagery 2011-2012. These mapservices can be opened in simple applications or mixed and matched with any other mapservices listed at ArcGIS Online into ArcGIS Online template applications.
ArcGIS Online divides content into:
Layers - a listing of a mapservice or data layer uploaded
Maps - one ore more Layers combined, possibly zoomed in to a certain area
Apps - template applications or links to other applications that include Maps
2) Catalog of applications and documents
ArcGIS Online allows organizations to post links to other information. MassGIS has listed our web page, OLIVER, and this wiki.
3) Template applications
ArcGIS Online provides simple template applications which mapservices can be plugged into easily.
4) ESRI Maps for Office
ESRI Maps for Office allows data in Excel or Powerpoint to be mapped using an ArcGIS Online mapservice.