In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers and that the speed of light in a vacuum was independent of the motion of all observers. This was the theory of special relativity. It introduced a new framework for all of physics and proposed new concepts of space and time.
Einstein then spent 10 years trying to include acceleration in the theory and published his theory of general relativity in 1915. In it, he determined that massive objects cause a distortion in space-time, which is felt as gravity.
Space-time is a sheet or grid and every time that mass is added to the sheet of space-time the sheet warps to the mass and creates a bend.
When more than one mass is added to the sheet of space-time there are attracted to each other but when they have a sideward force applied to them this is what is at play when things are orbiting other things. One earth when we show this it decays very quickly due to the forces but in space, it decays extremely slow due to the lack of force that can affect it.
A black hole is simply a star that imploded into its self and formed a black hole, this only happens if the star is big enough as if it smaller there will not be enough energy.
When you get close to a black hole time seems to slow down and then when you are in a black hole the time on the outside appears to be on fast forward.
In the centre of a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them to cease to operate.