There are indeed three types of NOTAMs.
NOTAM (D) - Used for enroute navigation...these notams would apply to navaid information, landing information, and most IFR related information
NOTAM (L) - Used for information only applying to a specific area/airport that is not (debatable) critical to flight. Information such as a taxiway closure or a runway barrier would be included in this type of NOTAM.
FDC NOTAM - Used for information pertaining to regulations, airspace, procedures, etc... For example the flight restrictions banning VFR aircraft right after sept 11 were issued in the form of a FDC NOTAM.
Ali