When you are trying to classify things, it helps to recognize the dominant characteristics that can gather a large mess of objects into smaller groups. In the case of animals, the idea of species creates some order in the animal kingdom. In particle physics one important invariant that helped divide the zoo into smaller groups was the idea of charge. How does the particle interact with the electromagnetic force? Electrons would bend one way, protons the other, and the neutron would be unaffected.
