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What is a Moon?
Moons, which are often also referred to as "satellites," are solid objects that are formed of rock, dust and gases circulating in space. In our solar system alone, astronomers have found at least 146 moons. These objects become trapped in the gravitational pull of a planet, such as Earth, and come to orbit that planet.
What is an Orbit?
The word "orbit" refers to the repetitive path taken by an object in space. When an object is in orbit, it is called a "satellite"--this is why moons are often referred to as satellites. A number of objects can be said to orbit. For instance, Earth has a moon that orbits the planet, but Earth itself orbits the sun. Human-made objects, such as telecommunications satellites, also orbit the Earth.
The Orbit of Earth's Moon
Earth's moon orbits the planet Earth on a steady elliptical path and at a more-or-less steady distance. The moon always faces Earth the same way since it is "tidal locked" to the planet, which means that its orbit is bound up with the Earth's tides due to the planet's gravitational pull; it completes its orbit every 27.3 days. The moon maintains an approximate distance from the Earth of about 365,000 kilometers, although it can drift up to over 400,000 kilometers away.
Other Moon Orbits
Every moon in the solar system orbits a planet--it is this orbiting that defines it as a moon in the first place. However, not all planets have moons. For instance, Mercury and Venus have no moon. The number of moons that a planet has can vary widely: Mars has two moons in its orbit, Jupiter has at least 50 and Saturn has 53, with nine more potential moons being studied as of 2011. These moons entered into the gravitational pull of their planets early on in the formation of the solar system.