A military base is a major installation, usually relatively permanent, that serves as an organizing and origination point for one or more branches of the military.
They usually have housing arrangements for enlisted and officers, both in barracks and family-style housing that closely resembles civilian housing developments.
An example would be a United States Air Force Base, which would have one or more runways, and a number of aircraft charged with some purpose, for which regular training would be engaged in. An air force base will often also be the "home" to an Air National Guard unit.
The United States has military bases in many parts of the world, to defend her interests (both commercial and political) and to honor her treaties.