Matter (solids, liquids or gasses) is all made up from particles.  Electrons, neutrinos and photons are all subatomic particles. A particle is something that is so tiny we can't see it.  Scientists are continuously investigating matter and the particles from which matter is made.  Atoms are so tiny you wouldn't even be able to see them with a good microscope - you would need a very special microscope.  But atoms aren't the smallest particles - electrons, protons and neutrons are even tinier.  Quarks even more so.