The term ready-to-fly or RTF refers to a fully assembled radio controlled model aircraft which requires little or no final assembly to operate.
RTF models generally are sold with necessary support gear in a single box. An engine-powered RTF requires the separate purchase of fuel; electrics are generally sold with everything except transmitter batteries. Inexpensive chargers are packed with models of this type as well. ParkZone and HobbyZone aircraft include eight heavy-duty AA batteries for the transmitter.
Most classes of aircraft, from balsa/ply trainers to high-performance foamies are available in RTF form.
RTF models are not without their critics, however. Though RTF's are generally well-constructed, many longtime modelers are blaming RTF models for a drop in the "hobby," or assembly phase of R/C aircraft modeling. On the other hand, RTF models are introducing a great number of individuals to the hobby who might not have the time, skill or desire to assemble a plane from the ground up. A crashed RTF is also far less of an emotional (and perhaps financial) loss as well.