Before discussing black holes themselves we should think briefly about gravity. Physicists recognize that the whole of the physical world can be described in terms of four basic forces. Two of these are concerned with the innermost structure of atoms, and a third, the electromagnetic force, dominates the interaction of atoms with each other. The fourth, and by far the weakest of these forces, is gravity. It is therefore only significant when enormous numbers of atoms are collected together into objects the size of the Earth, or bigger.
Gravity is the dominant force in the lives and deaths of stars and galaxies.
Any atom, or collection of atoms, has a property called mass, which measures how much material there is in the object. On the surface of the Earth, the gravity of the Earth pulls downwards on all masses giving the sensation of weight. On the surface of the Moon this pull, or weight, is only one sixth of that on the Earth, and so weight depends on where you are, whereas mass is an intrinsic property of all objects. In the 17th century Isaac Newton described gravity by saying that each mass attracts every other mass in the Universe with a force which depends on how much material is present and how far away it is. In 1915 Albert Einstein dramatically changed our idea of what gravity is, but Newton's description is adequate for this article.