Black hole are formed when stars collapse at the end of their life cycle. It is a region where nothing can escape even light. It is massive.

A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.
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When stars radiate energy like Sun for a long time(millions of years), the mass on the planet shrinks a lot and the temperatures becomes extremely small near 0 kelvin.  The density of the planet is so high that gravity becomes extremely large. no body can escape from that planet. Even light rays cannot escape from that planet. 
The effects of black hole can be felt from outside the planet.  the black hole star does not reflect light like moon or jupitar do - we are able to see them. Black hole is not seen.