A particle is a small piece of anything. Particle can also refer to a subatomic particle with mass and structure, but takes up hardly any space at all.
Particles are tiny. If you mean a subatomic particle, that's a body that you can't see because it is so incredibly small, though it does have a miniscule mass and internal structure. These particles are even smaller than atoms. You can also say a crumb or speck of something is a particle. Dust particles are piling up all the time, and vacuuming cleans up lots of particles. Those particles are little bits of things. In both cases, particles are very tiny.