Galaxies, including the Milky Way, exist ubiquitously throughout the observable Universe. They are immense conglomerations of stars, interstellar medium (ISM), and dark matter, all interacting through various physical processes. In general, galaxies consist of different populations, each formed at distinct epochs of the Universe. The sizes, masses, and luminosities of galaxies span several orders of magnitude. Their optical appearance (morphology) is primarily classified into three main categories: spiral, elliptical, and irregular galaxies. Spiral galaxies are further divided into two subclasses, and this overall classification is well described by the tuning-fork diagram. The luminosity of galaxies must also be considered in morphological classification, which leads to van den Bergh’s luminosity class (also known as DDO classification). Some galaxies exhibit peculiar properties, often explained by interactions and mergers, which can be incorporated into the classification scheme. The size of a galaxy is measured by its effective radius, within which half of the total luminosity is contained. The surface brightness distribution of galaxies varies with luminosity, complicating interpretation. This Introduction serves as a guide to more detailed discussions in the subsequent chapters.