According to the Nondestructive Testing Resource Center, a magnetic domain is the region in which magnetic fields of atoms are grouped together and aligned. In an unmagnetized object, all of the magnetic domains point in different directions. When it becomes magnetized, all magnetic poles align.
Magnetic domain structure is responsible for the magnetic behavior of ferrmagnetic materials, including iron, nickel and cobalt. Magnetic domains are separated by domain walls. In a magnetic domain, all electrons possess the same magnetic orientation on an atomic level. Magnetic domains of various forms are crucial in establishing permanent magnets, which are the common magnets in everyday objects.