ArcGIS for AutoCAD is a free of charge downloadable plugin from ESRI, currently available for AutoCAD 2007, 2008 and 2009.
With this functionality AutoCAD users have access to the ArcGIS Server Map Services and the desired map will be shown as an image behind the CAD-data.
The styling of the map is done by the server and the user can get info about the GIS-attributes.
This functionality is view only and doesn’t allow editting of the ArcGIS-data by AutoCAD.
AutoCAD Map/FDO is functionality to connect directly to for example an ESRI ArcSDE server and is available for AutoCAD Map 2006, 2007, 2009 and 2010.
With this functionality AutoCAD Map users can make a selection of the GIS-feature classes they would like to connect to in their CAD-environment. These are not displayed as an image, but as vector data. That makes it possible to “snap” to them in drawing mode.
The styling of the map can be done by using AutoCAD Map Display Manager. The GIS-attributes are available in a so called Data Table, where you can select a record to highlight the corresponding object in the map ( and vise versa ).
This functionality allows you also to edit the ArcGIS-data by AutoCAD Map.