In the Assign Coordinate System dialog, the Geographic (lat/long) category contains unprojected latitude and longitude coordinate systems. Coordinates are always in latitude and longitude. Each coordinate system has a different datum. Datums are defined for specific regions. Properties for each coordinate system and datum are listed in the dialog by clicking on the desired system.
See Also
Latitude and Longitude Coordinates
Latitude and Longitude in Decimal Degrees
Using Scaling to Minimize Distortion in Latitude/Longitude Projects