Black Holes are objects so massive that nothing can escape from them, not even photons of light. Stellar (or Galactic) BHs are the end products of the evolution of massive stars, and are typically 5-20 times as massive as our Sun. Supermassive BHs are about 1 million to 1 billion times as massive as the Sun; many say there's one residing the nucleus of every galaxy--including our own Milky Way. These supermassive BHs are suspected to be the cause behind active galactic nulcei (AGN) acvitvity. Black holes cannot be viewed directly (since no light can escape from them), but their gravitational influence on nearby stars and other matter is evidence for their presence.
 
 

Dark Spots of the Universe--Includes stellar evolution, how BHs are made, and what happpens in a BH's vicinity  Falling Into a Black Hole -- GIF movies illustrating observational effects  Spacetime Wrinkles--BHs and relativity  Click here for more related sites