The Oort cloud is a theoretical sphere of comets that lies roughly 0.79 light-years from thSun. The very outer edges of the cloud is thought to be the gravitational boundary of our Solar System.
It is thought to be made up of two parts, the spherical cloud that surrounds the Solar System, and a disk.
Long orbit comets that have orbits that last over 200 years are thought to be contained here. Although, the cloud is unconfirmed and no direct observations have been made, there are 4 objects that have been identified and are thought to lie in the cloud.
In the Oort cloud, the gravitational pull from the Sun is a lot weaker. Weak enough that the gravitational pull from the galaxies centre is no longer negligible (like it is everywhere else in the Solar System). This causes it to bulge in the same way that the Moon causes the oceans of Earth to bulge.