| "Neutron capture" is the name any of a series of nuclear reactions: (n,gamma), (n, p), (n, alpha), (n, 2n), and so on.
In these reactions, a free neutron collides with another atom and becomes absorbed, resulting in its transmutation to another nuclide (element or isotope).
Most common is the (n, gamma) process, in which a gamma particle is then emitted to carry away excess energy and the A number of the atom goes up by one (with no change in Z number. For example, a neutron can collide with a hydrogen atom to give a deuterium atom: 1H (n, gamma) 2H. This is the basis of neutron activation analysis -- the gamma particles emitted in this process are called "prompt gammas".
More interestingly, heavy targets can undergo the (n, p) process: instead of a gamma particle, a proton is "kicked out", resulting in a net decrease of the Z number by one and no change in A number.
The probability of such a reaction occuring is determined by the cross-section of the target for the particular energy of neutron. This is expressed in barns (1 barn = 10-24 cm2).
References: Nuclear and Radiochemistry, 3rd Ed., Gerhart Friedlander et. al. | 
