  Cosmic ray (CR) particles arrive at the top of the Earth's atmosphere at a
rate of around 1000 per square meter per second. They are mostly ionized nuclei
- about 90% protons, 9% alpha particles traces of heavier nuclei and
approximately 1% electrons. CRs are characterized by their high energies: most
cosmic rays are relativistic, having kinetic energies comparable to or somewhat
greater than their rest masses. A very few of them have ultra-relativistic
energies extending beyond 100 EeV (tens of joules). In this series of lectures,
delivered at the 2005 Mexican School of Astrophysics, an overview of the main
experimental characteristics of the CR flux and their astrophysical
significance is given. Particular emphasis is given to the upper end of the CR
energy spectrum. Unfortunately, due to space limitations, only a fraction of
the original content of the lectures is included in the present manuscript. In
particular, the production mechanisms are not included and the fundamental
topic of anisotropies is only dealt with in a very superficial way.
