AIM Text
Towers have been established to provide for a safe, orderly and expeditious flow of traffic on and in the vicinity of an airport. When the responsibility has been so delegated, towers also provide for the separation of IFR aircraft in the terminal areas.
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-1-2.
Research Notes
AIM 4-1-2 covers Control Towers — the visual ATC facilities that manage traffic on and immediately around an airport. The tower's job is to issue takeoff and landing clearances, manage runway crossings, and provide traffic information for arrivals and departures within Class B/C/D airspace surface area.
Tower vs TRACON distinction: The tower controls aircraft on the ground, in the pattern, and on initial departure/final approach. Once the aircraft is climbing out, the tower hands off to Departure Control (a TRACON facility, often physically located in the same building). On arrival, Approach Control hands off to the tower for the final landing clearance.
Class D vs Class C vs Class B: A Class D tower has no associated TRACON (or only minimal radar service); approaches are handled by the regional Center or a remote TRACON. A Class C tower has co-located radar approach control (TRACON). A Class B tower has even more sophisticated radar approach control with mandatory participation and separation services.
VFR pilot operating to a towered airport: The PIC must establish two-way radio communications with the tower before entering Class C/D airspace (§ 91.129, § 91.130). Communications are "established" when the controller has acknowledged the aircraft by call sign — see § 91.129 Research Notes for the "standby" nuance.
Light gun signals: When radio comms fail, the tower can issue clearances via colored light gun signals (steady/flashing green/red/white/alternating). Every pilot should be able to recite these from memory — they're tested on every checkride. See § 91.125 Research Notes for the full table.
Reference: AIM 4-2 (Radio Communications Phraseology); § 91.125 (ATC light signals); § 91.129 (Class D operations); § 91.130 (Class C operations).