Pulsars is the leftover core of a supernova explosion which rotates very rapidly. Pulsars emit radio-frequent pulses but the reason why is not that well understood. It is however thought that it has to do with the large magnetic field that is located at the base of the neutron star.
 
Pulsars are responsible for some cosmic rays but are not dangerous to Earth. The time between the pulses as roughly around a second as a rule but that is not foolproof in any way. The smallest approach about one millisecond. There are very few of those that are known at this date, they usually exist in binary star systems. Possibly a nearby stars infall of material causes the pulsar to speed up.

Pulsars after some time slow down and as a result of that, the pulses die out.
Pulsars were first discovered in 1967 during a radio astronomy observing program which was to look for "twinkling" radio sources.
 
For further reading visit:
http://cassfos02.ucsd.edu/public/tutorial/SN.html University of California, San Diego

http://www.herts.ac.uk/astro_ub/a41_ub.html University of Hertfordshire

http://www.pulsar.nl/ University of Hertfordshire