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.