Bosons, named after Satyendra Nath Bose, are particles which form totally-symmetric composite quantum states. As a result, they obey Bose-Einstein statistics. The spin-statistics theorem states that bosons have integer spin.
All elementary particles are either bosons or fermions.
Gauge bosons are elementary particles which act as the carriers of the fundamental forces.
Particles composed of a number of other particles (such as protons or nuclei) can be either fermions or bosons, depending on their total spin. Hence, many nuclei are in fact bosons. While fermions obey the Pauli exclusion principle: "no more than one fermion can occupy a single quantum state", there is no exclusion property for bosons, which are free to (and indeed, other things being equal, tend to) crowd into the same quantum state. This explains the spectrum of black-body radiation and the operation of lasers, the properties of liquid Helium-4 and superconductors and the possibility of bosons to form Bose-Einstein condensates, a particular state of matter.
Examples of bosons: