The Oort Cloud is a massive spherical cloud; the size of this cloud is disputed by different astronomers. Some believe that it begins at 2000 or 5000 astronomical units–an astronomical unit (AU) equals the distance between the Earth and the Sun—and ends at 50,000 AU, which is almost a light-year. Others think that it may extend to over 100,000 AU, which would mean its edge would extend to nearly the end of the Solar System.
The sphere was named after the astronomer Jan Oort who hypothesized its existence in 1950. Although its existence has not yet been proven through direct observation, the reality of the Oort Cloud is widely accepted in the scientific community. The Oort Cloud is filled with icy objects composed of ammonia, water and methane.
Although a few comets in our solar system come from the Kuiper Belt, most are thought to originate from the Oort Cloud. In fact, the Oort Cloud is composed of the same substances that comets are. The comets are formed when passing stars knock some of the objects in the Oort Cloud out of their orbits. These long-period comets then travel into the inner solar system. Short-period comets have orbits that last up to two hundred years while the orbits of long-period comets can last for thousands of years. Most short-period comets come from the Kuiper Belt, while long-period comets come from the Oort Cloud. There are a few exceptions to this rule though. Halley's Comet was a short-period comet that originated in the Oort Cloud.
The contents of both Kuiper Belt and the Oort Cloud are known as Trans-Neptunian Objects (TNOs) because the objects of both regions have orbits that that are further from the Sun than Neptune's orbit is. Because the Oort Cloud is so much farther out than the Kuiper Belt, it has not been as fully explored as the Kuiper Belt. Additionally, astronomers have been unable to identify objects to the degree that they have in the Kuiper Belt. In fact, aside from long-period comets, astronomers have only found four objects that, from the objects' orbits, they believe came from the Oort Cloud. Unfortunately, there is no chance of scientists examining the Oort Cloud up close and proving its existence anytime soon. Only recently was a spacecraft launched to examine the Kuiper Belt—NASA's New Horizons spacecraft—so it'll most likely be decades before any craft can be sent to the Oort Cloud.