Black holes have the gravity of about 2 quadrillion grams per cubic centimeter and in principle, any object - even a rock - can be made into a black hole, by squeezing it into a tiny enough volume. Under these conditions, the object continues to collapse under its own weight, crushing itself down to zero size. However, according to Einstein's theory, the object's mass and gravity remain behind, in the form of an extreme distortion of the space and time around it. This distortion of space and time is the black hole.
The resulting black hole is the darkest black in the universe: No matter how powerful a light you shine on it, no light ever bounces back, because the light is swallowed by the hole. A black hole is a true "hole" in space: Anything that crosses the edge of the hole - called the "horizon" of the hole - is swallowed forever. For this reason, black holes are considered an edge of space, a one-way exit door from our universe; nothing inside a black hole can ever communicate with our universe again, even in principle.
However black holes are even stranger than that. As you get closer to a black hole, the flow of time slows down, compared to flow of time far from the hole. (According to Einstein's theory, any massive body, including the Earth, produces this effect. Earth's gravity is so weak that the slowing of time is not noticeable, but the effect has been confirmed using sensitive instruments. For example, at sea level you age one-billionth of a second less every year than you would if you lived on top of Mt. Everest.) Near a black hole, the slowing of time is extreme. From the viewpoint of an observer outside the black hole, time stops. For example, an object falling into the hole would appear frozen in time at the edge of the hole.
At the very center of the black hole is where our understanding breaks down. Einstein's theory of gravity seems to predict that time itself is destroyed at the center of the hole: time comes to an abrupt end there. For this reason, a black hole is sometimes described as the "reverse of creation." But no one knows how or why time could come to an abrupt end, any more than we know how time was created in the first place. Einstein's theory of gravity no longer applies at these tiniest scales of distance, and new laws of nature must be found that describe what happens at the center of a black hole.
Does the inside of a black hole lead to another universe, as some scientists have claimed? The truth is, no one really knows. We cannot do a direct experiment to find out, even in principle, since no information or evidence can ever get back out of a black hole. That's what makes it so important to find and study black holes from the outside, while at the same time developing theories that can more confidently predict what might happen on the inside of the black hole.
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.
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.
Black holes can engulf other black holes. When they do, it causes a blast that is one of the most powerful blasts since the big bang.
An accretion disc is a disc of matter around a black hole that is spinning very fast around a black hole. The matter doesnt go in the black hole because it is spinning so fast around the black hole, it doesnt go in.