Pulsars are rotating neutron stars whose observed emission is pulsed with the rotation frequency of the star (up to
716 Hz as of 2007). Anthony Hewish and Jocelyn Bell discovered the first radio pulsar in 1967 (PSR J1921+2153). Since then nearly two thousand pulsars have been discovered from their pulsed radio and x-ray emission. This site describes some of the puzzling behavior of pulsars and many of the most interesting objects.
Not all neutron stars are observed to be pulsars. Whether or not the emission from these objects is inherently pulsed but we are unable to observed due to geometry is up to speculation. These objects share many of the observational properties of pulsars and pulsars eventually stop pulsing. The non-pulsar neutron stars are an exciting area of research as well.