Short answers, pulled from the story.
James Chadwick discovered the neutron in 1932 at the Cavendish Laboratory in Cambridge. He showed the new radiation consisted of uncharged particles with about the same mass as the proton, and won the 1935 Nobel Prize in Physics for the discovery.
A neutron is composed of three quarks: two down quarks and one up quark. This makes it a composite hadron and a baryon rather than an elementary particle, with the quarks held together by the strong force carried by gluons.
A free neutron spontaneously decays with a mean lifetime of about fifteen minutes. It decays into a proton, an electron, and an electron antineutrino through the weak interaction.
In December 1938, Otto Hahn, Lise Meitner, and Fritz Strassmann discovered nuclear fission induced by neutron bombardment. Because fission releases neutrons that can trigger further fission in a chain reaction, this led to Chicago Pile-1 in 1942 and the first atomic bomb at the Trinity test in July 1945.
Neutrons bind with protons and one another via the nuclear force, moderating the electromagnetic repulsion between protons and stabilizing the nucleus. Any nucleus with more than one proton requires neutrons, and heavy nuclei need extra neutrons to remain stable.
Fast neutron therapy uses high-energy neutrons typically greater than 20 MeV to treat cancer. Boron neutron capture therapy gives the patient a boron-containing drug, then bombards the tumor with low-energy neutrons that the boron-10 isotope captures, producing lithium-7 and an alpha particle that kills the malignant cell.
Neutron absorption does not increase straightforwardly with atomic number, so dense high-atomic-number materials like lead do not shield well. Hydrogen-rich materials such as concrete or paraffin are used instead because hydrogen scatters and slows neutrons, which can then be absorbed by an isotope like lithium-6.