Military airspace is any area in which military aircraft are present and participate in a variety of activities. There are many kinds of airspace.
Military Operating Areas (MOA) are zones in which military aircraft conduct non-hazardous exercises. It is highly recommended that pilots check for information on the MOA before proceeding into the zone.
Controlled firing zones are operated in such a way that they do not interfere with and/or pose a threat to civilian aircraft.
Alert areas are zones where unusual air exercises like pilot training are performed. Once again pilots are advised to contact the area prior to entering.