It is often loosely described as the smallest building block of matter, and from which elements are composed.
The main sub-atomic particles are the positively charged protons and neutral neutrons which occur in the nucleus of the atom and make up its mass, with the negatively electrons, being about 1600 times lighter than protons and neutrons, circulating the nucleus in different orbits and energy levels and constituting its volume.
The net charge on the atom is usually neutral, but when it is charged, it is quantized and integral multiples of the smallest charge, the charge on 1 electron being #1.6xx10^(-19)C#.
The energy possessed by each sub-atomic particle, which may be considered to have a dual nature, both particle and wave, is also always quantized. (#E=hf#)
However, if one studies quantam and particle physics, we see that there are also other smaller sub-atomic particles which can occur, and which may be entangled and which may not always be present in a predictable place and can quantam leap to different spots in their waveform. Examples would be quarks, gluons, muons, photons, leptons, mesons, bosons, etc.