A neutron is a subatomic particle; it is one of the building blocks of the atom. It has a mass of about 1.675 x 10-27 kg. Its spin is + 1/2 and that makes it a fermions. Additionally, it has no electric charge, which is a distinctive feature. It is unstable when free in nature, and has a half life of about 886 seconds.
The neutron could be said to be only "alive" to be part of an atomic nucleus as it ceases to exist after a while if left alone. When it wanders around loose, like after its release following decay event or a fission event, it may bump into another atomic nucleus and become captured by it. This process is called - no surprise - neutron capture. It is, after all, a nucleon, as is a proton, both of which make up an atomic nucleus. The neutron is made up of two down quarks and an up quark. When a neutron decays, it releases a proton (or, if you prefer, a hydrogen nucleus), an electron, and an antineutrino.   _{Your Comment}