There are 6 kinds of quarks and 6 types of leptons. So why is it that all the stable matter of the universe is only made from the two least massive types of quarks, up and down, and the least massive charged lepton, the electron?

Weak interactions are responsible for the fact that all the more massive quarks and leptons decay to produce lighter quarks and leptons. When a particle decays, it disappears and in its place two or more particles appear. The sum of the masses of the produced particles is always less than the mass of the original particle. This is why stable matter around us contains only electrons and the lightest two quarks (up and down.)

When a quark or lepton changes type (i.e. a muon changing to an electron) it is said to change flavor. All flavor changes are due to the weak interaction.

The carrier particles of the weak interactions are the W⁺, W^-, and the Z bosons. The W's are electrically charged and the Z is neutral.

A very interesting aspect of the Standard Model is that electromagnetic interactions and weak interactions are combined into a unified interaction called electroweak.