A neutron bomb is a small thermonuclear device designed to kill armored enemy troops with radiation while doing minimal damage to the surrounding area. It gets its name from the source of the lethal radiation -- energized free neutrons. These radioactive bullets can penetrate several feet of earth and practically any form of vehicle armor. This is in contrast to conventional nuclear weapons that emit lots of heat and light, which is blocked comparatively easily relative to radiation.
Neutron bombs are also known as enhanced radiation bombs (ERBs). The bomb was invented by physicist Samuel T. Cohen in the early 1970s. The intent of the bomb was to make a "nicer nuke" that did its job of eliminating enemy troops with minimal collateral damage to civilians or infrastructure. The Pope was so excited by the neutron bomb that he gave Cohen a peace medal in 1979. Another "ethical" aspect of the bomb is the low quantity of enduring radioactivity it leaves in its wake.
Among other thermonuclear weapons, the blueprints for neutron bombs were allegedly stolen by a spy working for the People's Republic of China in the mid-1990s. The motive for creating the neutron bomb comes from the days when missiles and artillery shells were not as precise, meaning that collateral damage from conventional warfare was likely. Nowadays, we have smart bombs and precise systems that hit their targets with an error factor of less than a few meters. This largely obsoletes the neutron bomb.
A neutron bomb with explosive force equal to one kiloton of TNT will kill any human at distances of a few kilometers or miles. At distances under 1 km (0.62 miles), the radiation is so strong as to immediately incapacitate the victim. This is about ten times the threshold at which half the exposed persons die. Those located further from the epicenter are also doomed to perish, but the process can drag on for days.
The neutron bomb's radiation also impregnates itself into surrounding materials such as the alloys found in armor. Exposed tanks have to be classified as low-level nuclear waste and disposed of accordingly. Samuel Cohen has since expressed shame for creation of the bomb. Hopefully it will never be used.