A neutron star is a mass entirely of neutrons the result of gravitational collapse of a star after it has used up the energy to resist such collapse. Its mass is large enough that gravity overcomes electron degeneracy and causes such collapse but not so large as to overcome their internal pressure which prevents them collapsing into a black hole (neutron degeneracy pressure). Typical mass is 1.4 to 3.2 solar masses with a radius on the order of 10 km. Stars with lesser mass become white dwarves instead.
Their existence was first proposed in 1934 and candidates were identified in the 1960s that have panned out.
Rotation of neutron stars and their magnetic field makes them pulse (pulsars). The magnetic field may be strong enough to affect the trajectory of accretion, channeling it to the magnetic poles. The point where the magnetic field is sufficiently strong to overcome the accreting mass's kinetic energy is called the Alfvén radius.
They are more complex than a mere "mass of neutrons" as the pressures at different depths produce different states of matter at different densities. They are thought to include iron at the surface, with something more like a simple mass of neutrons deeper, at which depths the material gains superfluid (no friction) and superconducting fluid (no electrical resistance) characteristics.