David Maguire has prepared an excellent overview the explains some of the functionality of the ArcGIS Explorer application over at his blog GIS Matters.

A couple of items of interest include:

• It will also be possible to view ArcIMS and OGC WMS services.
• Some more detailed and specialized services will be available for a fee.
• A new set of explorer services has been added to the Server core so that anyone (with ArcGIS Server) will be able to create and publish a service.