This chapter stars by showing that not only is it difficult to define a galaxy, but it is not helpful, given the overlapping masses of small galaxies and large star clusters. It is, however, useful to describe the properties of galaxies so that we can recognize one when we see it! The general properties of galaxies are described in terms of their mass components, from the most massive (the as yet mysterious dark matter) to the least massive (cosmic rays), and taking in the stars, and the interstellar gas and dust. A section on searches for dark matter candidates is followed by a section on alternatives to explain why the outer parts of disc galaxies rotate so quickly. The main explored alternative, Modified Newtonian Dynamics is mentioned, and we are shown why modifying gravity is unlikely to explain the observations. Next follows a description of the main types of galaxies: spirals, ellipticals, and irregulars, and their classification. Working downwards in mass components, we find star clusters and their main types: globular clusters, generally the more massive, and open clusters, less massive and younger. To complete the main picture we read about the interstellar gas, in its three main forms: molecular, atomic, and ionized hydrogen. The relations between the interstellar gas and the stars, which form from the gas and go on to inject heavy elements into interstellar medium, are then described. The supermassive central back holes are the most massive single objects in galaxies, but general contribute less than 1% of the total mass; their impact on their surroundings is, however, considerable, as explained. Many, but not all galaxies have central black holes. The mass component description is rounded off with a short description of cosmic rays.