Black holes are the result of dying stars that have at least 20 times more massive than the Sun. For most of a star's life, gravity and pressure
created by it's nuclear reaction balance each other exactly, and so the star is stable. However, when a star runs out of nuclear fuel, gravity gets the upper hand and the material in the core is compressed even further. The more massive the core of the star, the greater the force of gravity that compresses the material, collapsing it under its own weight.
A massive star may undergo a supernova explosion, leaving behind a massive burned-out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a "singularity." Around the singularity is a region where the force of gravity is so strong that not even light can escape. Thus, no information can reach us from this region. It is therefore called a black hole, and its surface is called the "event horizon."
Gas and dust from the region around the black hole will be pulled toward the black hole.
As the gas and dust are pulled toward the black hole they begin to orbit around the event horizon and then orbit the black hole. The gas becomes heavily compressed and the friction that develops among the atoms converts the kinetic energy of the gas and dust into heat, and x-rays are emitted. Using the radiation coming from the orbiting material, scientists can measure its heat and speed. From the motion and heat of the circulating matter, we can infer the presence of a black hole. The hot matter swirling near the event horizon of a black hole is called an accretion disk.
This is what a "Black Hole" may look like
Simulation of a trip to a "Black Hole"
Simulator courtesy of NASA and STScI