A neutron bomb is a nuclear weapon that releases a larger than usual fraction of its yield as neutron radiation. 20th-century human versions have nearly the same blast and thermal effects as a normal tactical nuclear weapon, but the radiation can kill enemy inside hardened targets (e.g. tanks) that would otherwise have survived. Alien versions may release a much higher fraction of their output as neutrons, killing organic life in a wide area without producing a large explosion.
On Skaro, neutron bombs were used in the neutronic war between the Dals and the Thals. (DW: The Daleks) These eventually caused the Humanoid Daleks to mutate into the "machine Daleks". (DC: Genesis of Evil)
The War Lords' Security Chief decided to use a neutron bomb to destroy the Humans' resistance movement, but the Doctor saved them by transporting them in a SIDRAT to Central control. (DW: The War Games)
Neutron bombs were radioactive weapons that poisoned a planet with extreme radioactivity. They were primarily used to severely reduce enemy resistance on a planet, while allowing Imperial troopers, mainly radtroopers, to capture a vital building or resource. Due to the severity of radiation after a neutron bomb, unprotected life forms would not be able to enter the bombed area for quite some time. The use of neutron bombs after the fall of the Empire is unknown.