Pulsars and Magnetars are both similar in origin as pulsars are essentially just a rotating neutron star and Magnetars are presumed to be former neutron stars. However, Magnetars have a immense magnetic field even when compared to Pulsars. What it boils down to is that the rotation and other factors allow for a much stronger gravitational pull. Quasars are completely different from both Pulsars and Magnetars, they are believed to be the bright centers of a galaxy. They release tremendous amounts of energy and are formed around supermassive black holes. To put it simply it's a -blam!-load of gas surrounding a huge black hole.