A pulsar pulses because its magnetic field is not perfectly aligned with its rotational axis. The intense magnetic field directs energy radiated by the neutron star and its surrounding accretion disk into a pair of jets which emanate from the magnetic poles. Since the magnetic poles are not aligned with the rotational axis, they appear to move as the neutron star rotates. If the star's rotation brings a jet in line with an observer, the observer will see a brief pulse of energy.

The rotation rate of a pulsar decreases with time, as do the magnetic field strength and the intensity of the radiation. High-energy radiation (such as gamma rays) is only produced by relatively "young" pulsars.
Source(s):
http://imagine.gsfc.nasa.gov/docs/science/know_l2/pulsars.html

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