The Oort Cloud is a theorized “shell” of comets and other objects that are on the edge of our Solar System. It is much harder to observe than other objects in and around our solar system like the Kuiper Belt, so it’s existence is only theorized, but it is the best way for us to explain the origin of long-period (longer than 200 years) comets. The Cloud itself is thought to be comprised primarily of ice and rock, the remnants of a disk of material that originally formed our planets and our Sun, and it is thought to exist between 10,000 AU and 100,000 AU away from our Sun. This distance means that the objects in the Cloud are very loosely bound to our Solar System, so they are often affected by the gravitational pull of passing stars and these gravitational effects can pull those objects in different directions and can even pull these objects out of the shell and the may begin “falling” toward our Sun. Information on the Oort Cloud is still fairly scarce since it is very difficult to observe and its very existence is still only theorized. There is a great deal of evidence for it, including, as stated previously, a number of comets as well as the planetoid Sedna, which is thought to be on the edge of the Cloud. It is thought that there are trillions of objects in the Cloud, so more evidence is discovered quite frequently.
The scale of the Oort cloud can be better seen here. To put things in even further perspective, one can consider the Voyager I space probe. ” At its current speed of about a million miles a day, NASA’s Voyager 1 spacecraft won’t reach the Oort Cloud for about 300 years. And it will take about 30,000 years to reach the other side.” [NASA].