  Pulsars are rapidly spinning neutron stars, that radiate at the expense of
their strong magnetic field and their high surface temperature. Five decades of
multi-wavelength observations showed a large variety of physical parameters,
such as the spin period, the magnetic field and the age, and of observational
properties, especially in the radio and X-ray band. Isolated neutron stars have
been classified according to the presence of thermal or non-thermal emission,
and whether they show a constant flux, rapid flares and bursts or long-standing
outbursts. One of the current challenges in the study of such objects is to
explain these different manifestations in the context of a unified evolutionary
picture. On the other hand, recent findings show that the classes of isolated
neutron stars are more connected than previously thought, and that non only
magnetars hold a complex magnetic field topology in the crust and above the
surface.
