The Oort cloud is an hypothesized spherical region at the outer limit of our Solar System that contains millions of small objects made from icy gas and dust.
It is thought that theses objects are the left overs from when the Solar System was created and that they were pushed to the edge of the Solar System by the combination of gravitational forces of it's planets and the Solar Wind.
Every now and then a nearby star will travel close enough to change the orbit of some of these objects, making them leave the Oort cloud and travel in towards the inner Solar System; where they will heat up, melt and create a trail of gases.
These objects are more commonly known as long-term comets, comets that have an orbital period of over 200 years. They typically have random orbits as though they have appeared from different parts of the Solar System supporting the theory the Oort Cloud is spherical in shape.