Comets come from a region known as the Keiper belt, an asteroid belt type area but populated with objects made up of mostly ice, and dust. Even though this belt is an incredible distance from the Sun, it is so massive it can still put pressure on objects here, especially if they crash into each other and are hit towards it. When this happens the comet will start to orbit the Sun, in an elongated and extremely eccentric way, sometimes taking hundreds or even thousands of years to complete just a single transit.