Radio controlled (R/C) airplanes come in many different shapes and sizes.  The smallest are powered by 2 stroke, glow fuel powered engines with displacements down to .010 cubic inches. They may weigh less than a pound and have wingspans barely over 12 inches.  On the other end of the spectrum are 4 stroke, glow and gasoline powered monsters with wingspans that can be over 12 feet and may weigh 50 lbs or more.  There are even model jets with real turbine engines in them. The cost of these models (with engine and radio) can range anywhere from just under $100 to several thousand. Materials to construct model aircraft may also vary widely.  Some are built with woods such as balsa and spruce and others with fiberglass, carbon fiber, and other hi-tech materials.  
A typical beginners model is built with wood and covered with a plastic film.  The wingspan is 50 to 60 inches and a .40 to .45 cubic inch glow engine is used for motivation. The model will probably weigh between 4 and 6 lbs. A four channel radio is used for guidance.  Each "channel" controls a function and they are throttle, ailerons, rudder, and elevator. Beginner models usually have very stable flying characteristics and should be capable of slow flight to make those first landings easy.  A beginners model will normally be able to do mild aerobatics such as loops and rolls also. Once the beginner has learned to fly they will probably want to graduate to either more nimble or scale like aircraft. Some models are designed to be aerobatic, others for speed, and yet others to replicate full scale aircraft.
One of the most frequently asked questions by visitors to the flying field is "how far away can it go?" The answer to that question is usually "as far away as I can safely see it". Model airplane radio systems are capable of controlling an airplane over 2000 feet away in most cases.  Another question is usually "how long can it stay up?".  A typical sized fuel tank will hold enough for a 15 to 20 minute flight. Finally, everybody wants to know how fast they go.  A slow model may only go 30 to 40 mph but some of the jets and racers can go over 200 mph.

If all of this sounds interesting to you then you're going to need more information on how to get started in model airplanes!