USAviation.info explains the subject in plain language. For flight, maintenance, certification or regulatory decisions, use the current official source and applicable approvals, procedures and guidance.
Controlled and uncontrolled airspace
The FAA groups controlled airspace into Classes A, B, C, D and E. Class G is uncontrolled airspace. “Controlled” does not mean every VFR aircraft is continuously controlled; it means ATC services and operating requirements are defined for that airspace. IFR operations in controlled airspace require an IFR flight plan and an ATC clearance.
Classes B, C and D
Class B requires an explicit ATC clearance before entry. Class C and Class D generally require two-way radio communication to be established before entry. Pilots remain responsible for meeting the applicable weather, equipment and operating rules for the airspace they enter.
What Tower, TRACON and ARTCC do
Airport traffic control towers manage airport traffic and the immediate terminal environment. TRACON facilities provide terminal radar approach/departure services around busy metropolitan areas. Air Route Traffic Control Centers (ARTCCs) handle en route traffic across much larger geographic areas. USAviation.info maintains a searchable directory of FAA ARTCC and dedicated TRACON facilities.
ATC service does not replace pilot responsibility
Radar advisories and sequencing do not relieve a VFR pilot of see-and-avoid responsibilities, required terrain and obstruction clearance, or compliance with applicable VFR weather minimums. Safety alerts are a core ATC service, but the pilot remains responsible for the safe operation of the aircraft.