Stars observed in galaxies were originally divided into two populations by Walter Baade in the 1940s. Although a more refined means of classifying stellar populations has since been established (according to whether they are found in the thin disk, thick disk, halo or bulge of the galaxy), astronomers have continued to coarsely classify stars as either Population I (Pop I) or Population II (Pop II). They have even postulated a third population (Population III; Pop III), though stars of this type have yet to be observed.
The classification system is based on the metal content of the stars (their metallicity, usually given the symbol [Z/H]). Pop I stars are the most metal-rich, with metallicities ranging from approximately 1/10th to three times that of the Sun (i.e. from [Z/H]=-1.0 up to [Z/H]=+0.5). This means that the gas from which Pop I stars formed must have been recycled (incorporated into, and then expelled) from previous generations of stars a number of times, and that Pop I stars are relatively young compared to Pop II and Pop III stars.
The Sun ([Z/H]~1.6) is a fairly typical Pop I star, as are most of the stars in the immediate solar neighbourhood. In fact, the majority of the stars contained within the thin disks of galaxies are Pop I stars, but Pop I stars can also found in the bulge.
The chemical composition of many Pop I stars in the bulge show higher abundance ratios for the lighter elements (e.g. carbon and oxygen). These elements are produced primarily in Type II supernova explosions (the explosions of massive stars), and indicate that the Pop I stars in the bulge formed early in the star formation history of the Milky Way. In contrast, the chemical composition of Pop I stars in the disk reveal that these stars are also enriched in the heavier elements that can only be produced in Type Ia supernova explosions (the explosion of a white dwarf in a binary system). This indicates that Pop I disk stars formed at least a billion years after star formation began in the Galaxy.