A millisecond pulsar (MSP) is a star (or other entity) which radiates with varying power - oscillating regularly at around 25Hz or more. Some have been found to vary at more than 700Hz. They have been discovered in the radio, X-ray, and gamma ray ranges of the electromagnetic spectrum.
Millisecond pulsars are thought to be related to low-mass X-ray binary systems. It is thought that the X-rays in these systems are emitted by the accretion disk of a neutron star produced by the outer layers of a companion star that has overflowed its Roche lobe. The transfer of angular momentum from this accretion event can theoretically increase the rotation rate of the pulsar to hundreds of times per second, as is observed in millisecond pulsars.
However, there has been recent evidence that the standard evolutionary model fails to explain the evolution of all millisecond pulsars, especially young millisecond pulsars with relatively high magnetic fields, e.g. PSR B1937+21. Bülent Kiziltan and S. E. Thorsett showed that different millisecond pulsars must form by at least two distinct processes. But the nature of the other process remains a mystery. Source: Wikipedia
Also see: Gamma-ray bursts