An orrery is a model of the solar system which is designed to show the locations of the various planets and their relationships to each other. Using internal clockwork and gearing, the orrery can be made to move, demonstrating the rotational patterns of various bodies in the solar system. These devices are sometimes called planetariums, which can be confusing for people who associate planetariums with large domes onto which various images are projected.
Archaeological evidence suggests that the concept of the orrery is quite ancient; the Greeks, for example, made several devices which resemble orreries. Production of these devices really took off in the 18th century, when a famous orrery was commissioned for Charles Boyle, the Fourth Earl of Orrery, and people began to take more interest in the solar system and the objects which inhabit it. A high quality orrery today can fetch a high price, as the construction techniques required to build one are quite demanding; a handful of artisans around the world continue to make orreries in their workshops.
It may come as a surprise to learn that many orreries do not include all of the planets. The Sun is almost always represented, typically as a large ball in the center of the orrery, but the included planets and moons vary widely. The Earth and Jupiter are commonly included, along with Mars, Mercury, and Venus, but the other planets may or may not be shown, depending on the intended use of the orrery and the taste of the person who built it. In a very complex orrery, moons may be included as well; this drives the cost up significantly, as the moons must be made to orbit the planets as the planets themselves orbit the sun.
Orreries are not usually built to scale, because the relative sizes of the objects in the solar system make this prohibitive, as do their comparative distances. However, it is typical to make the orbits as accurate as possible, so that while the planets may not be the right size or the right distance from each other, they will complete their orbits accurately, allowing people to see how and when the planets interact with each other.
Some classrooms have simple orreries for demonstrating basic topics in astronomy, although they more commonly use stationary models which must be moved by hand to illustrate things like eclipses and transits. It is also not uncommon to see an orrery at a science museum or astronomy center, and examples of antique orreries can be found in some museums.