Just like the land and oceans, the sky is divided into regulated regions. This makes sense, as it prevents unauthorised flights over sensitive and/or dangerous areas like airports, military zones, power stations, private land etc. Knowing the airspace classification is a fundamental prerequisite for making safe and legal flights with an unmanned aerial system (UAS).
In the UK, the Civil Aviation Authority defines airspace classes from A to G. There are specific permissions and restrictions associated with each class and they are mapped on VFR charts, for example here.
Class A airspace is the most heavily restricted and is less relevant for small UAS operators because only aircraft operating under IFR (instrument flight rules) are permitted to fly – limiting users mainly to commercial and private jets. Generally Class A starts from 18000 feet above mean sea level.
There is no Class B airspace in the UK, but it is commonly used to restrict airspace around large airports in the US.
Class C airspace usually extends vertically from 19,500 feet to 60,000 feet. It is permitted to fly using both instrument and visual flight rules (IFR and VFR) but clearance from air traffic control is necessary to enter. It is unlikely that a small UAS operator could end up in Class C airspace for several reasons, but especially because it would be very difficult to climb to 19500 feet!
Class D airspace is also available for VFR and IFR flights with clearance from air traffic control and at a speed less than 250 knots when flying below 10,000 feet. Typically the airspace around aerodromes (any location where flight operations occur) are Class D.
Class E airspace is also available for IFR and VFR use. Aircraft flying under VFR do not need clearance or two-way radio communication with air traffic control to enter but the pilot must comply with instructions from air traffic control.
Class G airspace is unregulated, meaning UAS pilots can fly as they please as long as the flight is within visual line of sight up to a maximum of 400 ft vertical and 500 m horizontal distance from the pilot in command and in accordance with the regulations set out in CAP 393 Articles 94 and 95 and CAP 722, for example being at least 50 m from any person, obstacle or vessel not under the direct control of the pilot, and at least 150 m from congested areas or open air gatherings of <1000 people.