A structure is made up of interrelated parts functioning as an orderly whole. It is synonymous to the definition of a system. It implies that a force is present to hold all these parts together. There are four kinds of forces in nature:
1 The gravitational force holds together massive objects in various astronomical systems and makes us earthbound.
2 The electromagnetic force operates on objects carrying electrical charges.
3 The weak force, which is responsible for beta decay of the nucleus, operates on microscopic objects in short range.
4 The strong force, which holds the nucleons together in the nucleus, is stronger than the weak force.
Text fragments from website: A review of the universe