R/C helicopters have only been around for a couple of decades but are an extremely popular part of the model aircraft hobby. Like airplanes, they come in many different shapes and sizes. The smallest on the market is powered by a .049 cubic inch displacement glow fuel engine but the most popular sizes are the .30 and .60 sized choppers. The average model helicopter costs more than a typical model airplane with an inexpensive heli, engine, and radio combo costing upwards of $600. Helicopters are usually made of plastic, metal, or composite materials and are more or less "assembled" as opposed to a model airplane in which there is much cutting, shaping, sanding, and gluing. A helicopter kit can be assembled in just a few days where as many model airplane kits may take months to build.
A typical first helicopter model will have a .30 to .60 cubic inch displacement engine and be guided with a 5 channel radio. The channels are throttle, left/right cyclic, fore/aft cyclic, main rotor pitch, and tail rotor pitch. Most model helicopter pilots use a gyro on the tail rotor pitch channel to help control the left/right motion (yaw) of the tail. A gyro senses any movement of the tail and inputs a correction to that channel to dampen the motion. Without a gyro the tail of the helicopter can move very suddenly and rapidly and is quite a challenge to control. The gyro not only makes it easier to fly but more enjoyable as well. Unlike airplanes, a beginner's helicopter can also be used as an advanced helicopter. Changing how sensitive the controls are is all that is needed to turn a tame and stable trainer into a very capable aerobatic machine.
Some helicopters have a no-frills look with a simple plastic canopy to cover the "mechanics" of the aircraft. Other models are completely scale with fancy fiberglass fueselages duplicating all the visual features of the real thing. A pilot can have either just by changing the canopy or fuselage and still keep the same basic airframe or mechanics.
Model helicopters, just like airplanes, can be flown as far away as the pilot can safely see them. They are capable of much more than full sized helicopters with loops, rolls, inverted flight, and so forth all within the range of the most common models on the market. It does however take a bit more skill to fly a model heli than it does an airplane. When hovering a helicopter you must not only worry about roll and pitch (as with an airplane) but also the left to right motion (yawing) of the tail. With a model airplane you never have to worry about the tail suddenly going out in front of the airplane. With helicopters it can happen which means you have to think about 3 directions at once instead of 2.
If all of this sounds interesting to you then you're going to need more information on how to get started in model helicopters!