Bosons are the particles responsible for carrying force in nature, including the photon for electromagnetism, the gluon for the strong force, and the W and Z bosons for the weak force. The Higgs boson, discovered in 2012, belongs to this family too, though it works differently, giving other particles their mass rather than transmitting a force.
What defines a boson is its integer spin, and a key quantum property that follows from it, any number of bosons can pile into the exact same quantum state at once. That is why photons can be packed together so densely in a laser beam without any restriction.
Bosons stand in contrast to fermions, the particles that make up matter itself, which are forbidden from sharing a state under the Pauli exclusion principle. The divide between the two is one of the deepest organizing principles in the Standard Model. For a full walkthrough of how they fit together, see Astrinova’s guide to particle physics.
The tendency of bosons to crowd into the same state has a dramatic real world consequence called a Bose-Einstein condensate. Cool a gas of bosonic atoms to within a fraction of a degree of absolute zero and, instead of behaving as individual particles, they collapse into a single collective quantum state that behaves like one giant matter wave. Physicists Eric Cornell, Wolfgang Ketterle, and Carl Wieman produced the first Bose-Einstein condensate in a laboratory in 1995, winning the 2001 Nobel Prize in Physics for it. The effect is named for Satyendra Nath Bose, whose 1924 work on photon statistics, later extended by Einstein, gave bosons their name and first revealed this strange tendency to congregate.
The statistical rules bosons follow, known as Bose-Einstein statistics, describe how identical bosons distribute themselves across available energy states, and they differ sharply from the rules fermions must obey. That single mathematical difference in how particles are counted is ultimately what separates force-carrying particles, which can pile together freely, from matter particles, which cannot.