A neutron bomb was a nuclear weapon that, according to the First Doctor, "destroy[ed] all human tissue, but [left] the buildings and machinery intact." It was also shown to destroy the tissue of other organic matter, such as Time Lords and Thals.
On Skaro, neutron bombs were used in the neutronic war between the Dals and the Thals. (TV: The Daleks) These eventually caused the humanoid Daleks to mutate into the "machine Daleks". (COMIC: Genesis of Evil)
The War Lords' Security Chief decided to use a neutron bomb to destroy the humans' resistance movement, but the Second Doctor saved them by transporting them in a SIDRAT to Central control. (TV: The War Games)