Lets start with what are Quarks and Leptons: Quarks and Leptons are the building blocks of matter.
Because they are so small they are normally referred to in cross section as that is a better way to understand them then by mass. But to give you an idea molecules are made of Protons, Neutrons and Electrons. One Proton is made of 3 Quarks. Quarks can be found in 6 different varieties and all have different characteristics or "colors"
But if you look at Leptons there are smaller particles called Neutrinos. These are referred to in experiments as: