In the observation and study of stars it is helpful, like in other branches of science, to subdivide the objects of investigation into different types. On the Earth we obtain information about a star from its apparent magnitude and its spectrum. Simply speaking, cool stars appear reddish, and hot stars bluish. A distinction then can be made between different spectral classes of stars. A brief overview of this topic and a summarizing look at the evolution of stars are the subject of this chapter.