There are actually three types of comets astronomers can distinguish once they have made enough measurements to compute their orbits. Short-period comets have elliptical orbits that bring them into the inner solar system (inside the orbit of Mars) every 200 years or less. Some of the famous short-period comets include Halley's (76 years) and Encke (3.3 years). These comets seem to have come from inside the orbit of Neptune, and there are enough of them that there must be quire a few of these comets-in-waiting ready to get bumped into an orbit that takes them deeper into the solar system. Jupiter is usually credited with stirring up comets the most. The next class is the long-period comet. These have very steep elliptical orbits that take them far beyond the orbit of Pluto and it is believed that this is where the reservoir for these comets occurs - possibly the Kuiper Belt. Orbit periods can be thousands of years long, and are at the limits of our ability to both detect (because they are rare) and because their orbits are so steep that it takes many separate measurements to really tie them down to such extremely elliptical paths. The last category could be informally called the 'Interstellars' which would have speeds so fast they must be on orbits that begin and end in the distant Oort Cloud, or perhaps even interstellar space. Any object seen with a speed greater than 90,000 miles per hour cannot be gravitationally 'bound' by the Sun. Many of these are believed to have been long-period comets that were sling-slotted by a close encounter with Jupiter into the orbits we now find them. So far, astronomers have not detected any of these visitors from interstellar space.
