Population is a way of classifying stars on the basis of several properties, including location in the host galaxy, type of orbit, and heavy element content or metallicity; it was introduced by Walter Baade in 1943. In Baade's scheme there are two main population types: Population I and Population II. In reality, there is continuous gradation of stars from one type to the other. A more refined system, based on modern knowledge, sees our Galaxy and others made of four stellar populations: thin disk, thick disk, stellar halo, and bulge.
The thin disk population is confined to within about 1,000 light-years of the galactic plane and includes the Sun and 96 percent of its neighbors. Thin disk stars are metal rich, vary in age from newborn to 10 billion years, and revolve around the Galaxy fast in fairly circular orbits.
The thick disk population, which probably includes Arcturus and about 4% of the Sun's neighbors, is generally older than the thin disk and extends several thousand light-years above the galactic plane. Its members move in elliptical orbits and have metallicities around one-quarter that of the Sun.
The halo population is a roughly spherical system of very metal-poor stars (1 to 10 percent of solar metallicity), mostly subdwarfs, that move in highly elliptical orbits that may reach up to 100,000 light-years from the galactic center at apogalacticon and as little as a few thousand light-years at perigalacticon. The nearest example of a halo star to the Sun is Kapteyn's Star.
The bulge population occupies the central few thousand light-years of the Galaxy and consists of old, metal-rich stars. No such objects are in the solar neighborhood; it is the least explored stellar population in the Milky Way.
Population III is a hypothetical population of stars which consist almost entirely of hydrogen and helium and virtually no heavy elements (in astrophysical terms, elements heavier than helium) in their surface layers. Population III stars are theorized to have been the first stars that formed after the Big Bang when there were no virtually no heavy elements in space apart from low concentrations of a few isotopes such as lithium 7 formed in the Big Bang itself.