Magnetism is a kind of physical phenomena or property that is mediated by magnetic fields. Electric currents and the magnetic moments of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. 

A bar magnet with iron filings indicate the direction of magnetic field. image: wikipedia/Newton Henry Black

Every material is influenced to some extent by a magnetic field. The most familiar effect is on permanent magnets, which have persistent magnetic moments caused by ferromagnetism

Most materials do not have permanent moments. Some are attracted to a magnetic field (paramagnetism); others are repulsed by a magnetic field (diamagnetism); others have a more complex relationship with an applied magnetic field (spin glass behavior and antiferromagnetism). 

Substances that are negligibly affected by magnetic fields are known as non-magnetic substances. These include copper, aluminium (although conductors can still produce an eddy current when exposed to a magnetic field), gases, and plastic. Pure oxygen exhibits magnetic properties when cooled to a liquid state.

The magnetic state (or magnetic phase) of a material depends on temperature and other variables such as pressure and the applied magnetic field. A material may exhibit more than one form of magnetism as these variables change.

Source adapted from:  Magnetism. (2015, August 9). In Wikipedia, The Free Encyclopedia. Retrieved 07:30, August 11, 2015, from