A pulsar is a rapidly spining neutron star, which is the dense cinder left over from a single star about 5 times the mass of the Sun, but probably less than about 10 times the mass of the Sun. These stars end their lives as supernovae, ejecting much of their mass into interstellar space, but this also causes the core of the star weighing less than 1.44 times the mass of the Sun, to implode into a body about 20 kilometers in diameter. Spinning at up to 30 times a second, its powerful magnetic fields accelerate electrons into beams of electromagnetic radiation which, if we happen to be lucky, sweep past us and cause the neutron star to give off pulses of radiation, usually at radio wavelengths, but sometimes visible at optical and even x-ray wavelengths. There are perhaps over 100,000 pulsars in the Milky Way galaxy of which only 800 are known as yet.
Quasars are enigmatic, luminous objects located billions of light years from the Milky Way galaxy at so-called 'cosmological' distances. They seem to be associated with the nuclear regions of some galaxies, but not all, and emit enormous fluxes of electromagnetic radiation. We believe that they are powered by very massive black holes, each with 1 million to several billion times the mass of the Sun, packed into a region of space smaller than our solar system. We do not know how super-massive black holes form, but the Hubble Space Telescope has detected these leviathan objects in several nearby galaxies. When matter or whole stars fall into these objects, as much as 45 percent of their rest masses are converted into pure energy via E = mc^2. This is why they can be so powerful, but occupy such a small volume in space. Even our own Milky Way may have a baby- quasar inside its core, weighing a few million times the mass of the Sun, and just waiting for a star or a gas cloud to feed it!