Here we will give an overview of the observed properties and chemical abundances of metal-poor galaxies, while in Sect. 5 we will discuss a few of these in more detail. As a metal-poor galaxy is in general a dwarf galaxy, this section will be biased towards dwarf galaxies.
Dwarf galaxies come in different kinds, with different properties and metallicities. There is a partial overlap in the classification and physics of metal-poor galaxies. At faint absolute magnitudes the dwarf irregular (dI), dwarf elliptical (dE) and low surface brightness galaxy (LSBG) classes converge. Moreover BCGs to some extent overlap with actively star forming dIs. The possible connections between different types of galaxies will be further discussed in Sect. 7.2. A lot of the data on dI and dE/dSph galaxies are from the Local Group. It is not our intent here to give a general description of the Local Group dwarfs, recent and excellent reviews can be found in e.g. Mateo (1998, which also contains a compilation of very useful data) and Grebel (1998); see also the book ``Stellar astrophysics for the Local Group'' (eds. Aparicio et al. 1998) and the proceedings of the recent IAU symposium ``The Stellar Content of Local Group Galaxies'' (eds. Whitelock and Cannon 1999). Here we will only focus on those points that are relevant for the understanding of the most metal-poor galaxies.
As we have mentioned, metallicity correlates with luminosity for galaxies, at least in the local Universe. More metal rich galaxies are on average more luminous and have in addition higher surface brightness. This produces a bias against detecting metal-poor galaxies. Moreover, except for H II region spectroscopy, most ways of measuring metallicity discriminate against low luminosity galaxies.