Original: While in a non-rotating black hole the singularity occurs at a single point in the model coordinates, called a "point singularity". In a rotating black hole, also known as a Kerr black hole, the singularity occurs on a ring (a circular line), defined as a "ring singularity".
Simplified: A gravitational singularity (sometimes called a spacetime singularity) is a place in a black hole where the gravity is thought to be infinite.