A fermion is any particle that has an odd half-integer (like 1/2, 3/2, and so forth) spin.
Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions.
For reasons we do not fully understand, a consequence of the odd half-integer spin is that fermions obey
the Pauli Exclusion Principle and therefore cannot co-exist in the same state at same location at the same time.
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Bosons are those particles which have an integer spin (0, 1, 2...).
All the force carrier particles are bosons, as are
any composite particles with an even number of fermion particles
(like mesons).
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