These lectures are meant as an introduction to some basic physical processes taking place in T Tauri stars. The first section is devoted to a brief description of the various optically visible objects encountered in molecular clouds, and also gives a short overview of early studies of young stellar objects. The second section shows how the spectroscopic criteria defining the class of T Tauri stars can be translated into relevant physical properties. The third section presents our current understanding of these stars, with emphasis on the role of their circumstellar disks. Because current knowledge of outflows is presented elsewhere in this book, no attempt is made here to discuss T Tauri winds in detail.