Commonly uses Abbreviations in R/C Industry Defined

ARF
Almost Ready to Fly.
Meaning that the model itself has for the most part been pre-assembled at the factory but may need a few components installed or purchased like a battery, battery charger, motor,  receiver, servos, or radio system. Some modelers may already have a radio system, receiver, motor, and  servos that they can utilize without spending additional money. Check the package details before purchasing to identify what is is included.
RTF
Ready to Fly
Meaning the model itself for the most part has been pre-assembled at the factory and includes all the necessary components to get the model off the ground and in the air. In most cases the servos, are pre-installed.May require some minor assembly.
This package normally includes the battery, charger, receiver, motor, ESC, and radio system. The battery will most certainly need to be charged before your ready to fly. Check the package details to insure you know what is included with the package.
PNP 
Plug N Play
Meaning the package includes a pre-assembled model from the factory with some of the components, but may require a radio system, receiver, and possibly the flight battery to get the model off the ground and in the air. Check the package details to insure you know what is included.
RTR
Ready to Run
Usually refers to R/C trucks, cars, and boats. Meaning that the R/C model is pre-assembled and ready to run but you may need a component like a battery or charger. Check the package details to insure you know what is included.
ARR
Almost Ready to Run
Meaning that the model has been pre-assembled at the factory, but may not include a battery, motor,  radio system or receiver.
Scale
Indicates the size of a model compared to its life sized equivalent. Like 1/4 , 1/8, 1/10, 1/16.
A 1/4 scale model is larger than a 1/16 scale model. A 1/10 scale model is 1/10 the size of the life size version.
2.4GHz
A particular frequency band that has replaced the older MHz radio systems. The 2.4GHz systems are not normally prone to interference for glitch free operation.