A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

 Black holes are interesting for many reasons: they are one of only three possible endpoints of stellar evolution (the others beingwhite dwarfs and neutron stars), they are the powerhouses of the most luminous things in the universe (quasars and active galactic nuclei), and they are the simplest macroscopic objects in the Universe, with only two parameters important for their astrophysical properties. 

Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that. 

Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.