  Cosmic rays represent one of the most fascinating research themes in modern
astronomy and physics. After almost a century since their discovery, a huge
amount of scientific literature has been written on this topic and it is not
always easy to extract from it the necessary information for somebody who
approaches the subject for the first time. This has been the main motivation
for preparing this article, which is a concise and self-contained review for
whoever is interested in studying cosmic rays. The priority has been given here
to well established facts, which are not at risk to get obsolete in a few years
due to the fast progress of the research in this field. Also many data are
presented, which are useful to characterize the doses of ionizing radiation
delivered to organisms living on the Earth due to cosmic rays. The technical
terms which are often encountered in the scientific literature are explained in
a separate appendix.
