An orbit is an object's revolution around another. Orbits are caused by the gravitational attraction between the two masses. The smaller mass is prevented from falling into the large mass by a large velocity perpendicular to the object it is orbiting around. The concept of orbiting in the solar system has changed drastically over time. Astronomers before Tycho Brahe and Kepler assumed that the orbits of observable planets were circular. Kepler was the first to propose that the planets' orbits were elliptical.
The orbit of the Moon is nearly a perfect circle, but due to its high relative mass the focus of its orbit (its barycenter) is actually closer to the Earth's surface than it is to the exact center of the Earth.
Comets, on the other hand, have highly eccentric orbits with only one focal point inside the Sun.