From an observational standpoint, what is a pulsar?

According to astronomy class notes from the University of Tennessee at Knoxville, a pulsar is a special kind of star cluster formed when two stars spin rapidly around each other, and flash their light in a way that is visible from earth. It is not well-known how pulsars work.

Scientists believe that the strong magnetic fields associated with the stars are responsible at least partially for the speed at which they spin around each other. If a beam of a pulsar hits Earth, the light from the pulsar appears to pulse on and off. There are several hundred known pulsars.