Cosmic rays are energetic particles coming from outer space. They are mostly made up of nuclei of various elements, together with small amounts of electrons, gamma-ray photons, and anti-matter particles. At the early stages, many particles were discovered in cosmic rays, such as positrons, muons, pions, which became the basis of the building of particle physics. The interaction between cosmic rays and matter is just similar with particle colliders, but it can reach much higher energies than even our largest colliders on the Earth, and can thus offer us a unique opportunity to probe physics at extreme energies. The origin, acceleration, and propagation of cosmic rays are also very interesting and fundamental questions in modern physics and astrophysics.