A magnetic field is an invisible field which exerts magnetic force on substances which are sensitive to magnetism. A classic example of a magnetic field is the field created by an iron magnet; to see how the energy in such a field works, you can place a small magnet under a piece of paper and sprinkle iron filings on it. As the filings respond to the magnetic field, they will slowly orient themselves along an axis. Larger examples of magnetic fields include the Earth's magnetic field, and the magnetic fields which are created by other celestial bodies such as stars and planets.

Many people think of magnetism as a property of metal, particularly iron, since common household magnets are made from iron. Electrical currents are actually the force behind magnetic fields, which form as electrical charges move around. On a large scale like an electromagnet, the magnetic field is created by passing current through wires. In the case of a household magnet, the field is created by the movements of electrons in their electronic orbits. Depending on the material and environmental factors, the strength of a magnetic field can vary.

Magnetic fields have numerous properties which scientists and others have utilized over the centuries. In navigation, ships can orient themselves with the assistance of the Earth's magnetic field, which is incidentally located several degrees off the geographic poles. In scientific research, magnetic fields can be used to gather information about a location or object; geologists, for example, use tools called magnetometers to measure ambient magnetic fields to learn more about the underlying rock and mineral materials.

Medicine has a use for magnetic fields, in things like diagnostic machines such as Magnetic Resonance Imaging (MRI) equipment. In this case, the magnetic field is carefully generated and controlled by the operator of the machine for the purpose of gathering information about the human body. In alternative medicine, some healers use magnetic fields to achieve various desired effects. Scientists can also use other tools to create or alter magnetic fields for the purpose of learning more about the Earth.

The strength of magnetic fields varies widely. An MRI machine, for example, can suck keys out of an unwitting doctor's pocket, while a household magnet can be knocked from the fridge with little effort. There are also numerous different types of magnetic fields; an iron magnet, for example, displays ferromagnetism, while magnetic fields created with the use of an external current are known as electromagnetic fields.