One of the strongest forms of magnetism which is the basis for all permanent magnets.
Ferromagnetism refers to the phenomenon by which ferromagnetic metals such as iron, nickel, cobalt and certain alloys become magnetized in a magnetic field and retain their magnetism when the field is removed.
Magnetization of a ferromagnetic material
For example, iron is made up small regions known as magnetic domains, each domain having a different magnetic orientation, so the material as a whole may not be magnetic.
When iron is placed in a magnetic field, the domains align in the direction of the field, thereby magnetizing it. Because iron is ferromagnetic, it retains its magnetism even when the field is removed.
Source: Salt Lake Community College
Magnetism in Matter