Any small scale representation of a full size airplane is considered to be a model airplane. A number of different varieties of model airplane exist for many different purposes: some are simple plastic models made to be showcased on home mantelpieces, while others are gas or battery powered versions that can actually fly. A model airplane can be made of simple plastic, die cast metals, or in the case of higher end versions made to fly via remote control, composite materials such as graphite or plastic that are then covered with airtight material and "doped" to make the plane aerodynamic.
The simplest version of a model airplane is the static scale model. These airplanes often come in kits and must be assembled using rubber cement or epoxy, and they might be made of plastic, balsa, fiberglass, paper, or other simple materials. They are popular among children and model hobbyists and are modeled after hundreds of different life-sized aircraft. Simple plastic model airplanes, like all other model planes, can also be made in the image of imaginary or concept aircraft as well. These types of model airplanes are sometimes used for promotional purposes as well, and are typically built to a certain scale. The most popular scale is 1:72--that is, 72 of the model airplane, placed end to end, would be the same length as one of the real airplanes.
Some versions of the model airplane are made to actually fly. There are generally three categories of flying model aircraft: a free flight version that is in no way attached to the ground or to the user, like a glider that is hand-thrown; a control line model plane, which uses a wire that tethers the plane to the user; and radio-controlled aircraft, in which the pilot uses a handset to send radio signals to a receiver inside the airplane. Constructing flying model airplanes is generally more labor-intensive and complex than constructing a static scale model, and flying models are often closer in form and function to real aircraft. Adding to the complexity of flying model aircraft, the planes almost all require some sort of propulsion in the form of an engine or combustion system, which can be tricky to build and maintain. Flying model aircraft can be built from scratch, but for entry level users, kits can be purchased that are completely or almost completely built. These are called ARF models, which stands for "Almost Ready to Fly."