A black hole is a region of space time exhibiting such strong gravitational effects that nothing - not even light - can escape from it. Black holes are measured in solar masses, with each unit representing the weight of the sun. They can be as small as the Planck length, while the largest are known as supermassive black holes. The latter weigh in at billion of solar masses. The following video by Times Infinity presents the top 5 biggest black holes ever discovered.
[Source: Times Infinity/YouTube.]