Physicists have developed a theory called the Standard Model that attempts to describe all matter and forces in the universe (except for gravity). Its elegance lies in the ability to explain hundreds of particles and complex interactions in terms of a few fundamental particles and interactions.

Force Carrier Particles:
Each type of fundamental force is "carried" by a force-carrier particle (an example of which is a photon.)

Matter Particles:
The Standard Model says that most matter particles we know of are actually composites of more fundamental particles called quarks. There is also another class of fundamental matter particles called leptons (an example of a lepton is the electron).

So, there are two kinds of particles:

• particles that are matter (like electrons, protons, neutrons, and quarks) and
• particles that carry forces (like photons).