Controlled airspace is airspace of defined dimensions within which Air Traffic Control (ATC )services are provided.The level of control varies with different classes of airspace. Controlled airspace usually imposes higher weather minimums than are applicable in uncontrolled airspace. It is the opposite of uncontrolled airspace.
Controlled airspace usually exists in the immediate vicinity of busier airports, where aircraft used in commercial air transport flights are climbing out from or making an approach to the airport, or at higher levels where air transport flights would tend to cruise. Some countries also provide controlled airspace almost generally, however in most countries it is common to provide uncontrolled airspace in areas where significant air transport or military activity is not expected.
Controlled airspace is in place to provide a safe area for aircraft operations around an aerodrome for landing and take-off and for aircraft enroute between two aerodromes.
Uncontrolled airspace is airspace where an Air Traffic Control (ATC) service is not deemed necessary or cannot be provided for practical reasons. According to the airspace classes set by ICAO, both class F and class G airspace are uncontrolled. It is the opposite of controlled airspace.
ATC does not exercise any executive authority in uncontrolled airspace, but may provide basic information services to aircraft in radio contact. Flight in uncontrolled airspace will typically be under VFR. Aircraft operating under IFR should not expect separation from other traffic: however, in certain uncontrolled airspace, this might be provided on an ‘as far as is practical’ advisory basis.
More airspace information.