  Magnetars are neutron stars in which a strong magnetic field is the main
energy source. About two dozens of magnetars, plus several candidates, are
currently known in our Galaxy and in the Magellanic Clouds. They appear as
highly variable X-ray sources and, in some cases, also as radio and/or optical
pulsars. Their spin periods (2-12 s) and spin-down rates (~10^{-13}-10^{-10}
s/s) indicate external dipole fields of ~10^{13-15} G, and there is evidence
that even stronger magnetic fields are present inside the star and in
non-dipolar magnetospheric components. Here we review the observed properties
of the persistent emission from magnetars, discuss the main models proposed to
explain the origin of their magnetic field and present recent developments in
the study of their evolution and connection with other classes of neutron
stars.
