A subatomic particle is any particle that is smaller than or makes up an atom. At one time this included protons, neutrons, and electrons, which are known to make up all elements (except ordinary hydrogen does not have any neutrons). Since that time additional particles have been experimentally detected or theorized. These include quarks and gluons which are a level smaller still.
Most of space on the subatomic level is empty, with only very small amounts having particles in them.
Subatomic particles are categorized into: