Magnetisation is the property of a body at the origin of a magnetic field. Magnetisation is a manifestation of several types of magnetism.
It originates from a density of microscopic magnetic moments carried by electrons within the material (microscopic currents imagined by Ampere): see the Bohr magneton.
There is permanent magnetisation (a magnet) and induced magnetisation caused by an external magnetic field or a current (induction, electromagnet).
The intensity of magnetisation is related to the magnetic field B and its excitation H by the relation: B = μo (H+M) , where μo is the permeability of a vacuum, multiplied by a relative permeability in a medium (Maxwell-Ampere equations).