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: "the results are consistent with zero mass for the neutrino".
This is all part of Quantum Physics. As I stated before this is a complicated question with varied answers. The physicists have more questions then answers about these small particles that cannot be seen only their effects on each other is observed.
Please continue to explore this field and ask more questions. Below are a few links that I used to help my understanding.
