The variety of coordinate, projection systems and datum available can be overwhelming to the user. National Geodetic Survey has a good FAQ . A more extensive explanation from RPI will probably provide with with more detail than you want. Patrick McGlamery is a librarian. There is no GIS position here, just a humble librarian.
Libraries provide access to material provided by a publisher.
Depending upon who the publisher is, the data are in that format, coordinate system, projection, datum and map unit. The ConnDEP's Environmental and Geographic Information Center has transformed all of there vector and raster data from NAD27 to NAD83. Conn. State Plane, NAD83. Conn. State Plane, NAD27. These two datums will display the data on the ArcView screen in very different places.