Black is a congealed mass of matter that has lost is atomic integrity. People think that black hole is an extremely dense solid object like a planet - this is a misconception. To understand what black hole is we need to understated its evolution.
This happens when intense gravitational forces pull the matter together so strongly that the electrons are absorbed by the protons (reverse beta decay) and thus the atoms exist no more and we have a giant condensed ball of neutrons packed together (thus the name: neutron star). At this stage we call it a neutron star.
If we keep on adding more matter to the this neutron star we will exceed a certain threshold (typically three times the solar mass), then this neutron star would further collapse into a black hole.
If neutron star evolves into a black hole, then nothing can escape its gravitational field and even light is absorbed by it. Thus we have the name 'black hole'. Thus this whole region of space and time from which escape of anything is impossible is called a black hole. The fact is that we cannot even really study this region called black hole as it's observation is impossible.
A black hole's existence is so shrouded in mystery that the only way to understand it is to observe how the black hole interacts with its environment.
NOTE: Black holes too have different classifications and characteristics.