An HII region (HII, pronounced as "H two region" or "H two") is a hydrogen cloud that is partly ionized hydrogen (HII). Typically their temperatures reach 10000K. They often exhibit a 656.3 nm hydrogen emission line (H-alpha of the Balmer series). Region sizes range from a fraction of a parsec to hundreds of parsecs across. The Orion Nebula and the Horsehead Nebula are examples of HII regions.
Other kinds of hydrogen clouds are HI regions and molecular clouds. HI regions and HII regions are termed diffuse clouds and molecular clouds are termed dense clouds.
HII regions are considered to be formed by hot, blue, early stars within them, such as OB stars, which produce ultraviolet, including photons of sufficient energy to ionize nearby hydrogen. A Strömgren sphere is a model of the process, for a single star. Larger regions are considered the result of more than one such star. Since such stars don't last long, HII regions are taken as a short-lived result of star formation, so the star formation must be recent.
As such, HII regions form within regions that trigger star formation, i.e., molecular clouds. At the border is a region where some molecules undergo photodissociation, i.e., a photodissociation region. It is thought that the radiation from the bright star(s) and the heating of the gas pushes and compresses surrounding molecules, leading to more gravitational collapse and resulting star formation within the molecular cloud.
HII regions have at times been used to determine distances to galaxies, though I believe this is now considered unreliable. The theory is of the existence of a HII region size—galactic luminosity relation, specifically, a relation between the luminosity of a spiral galaxy and the size of its largest HII regions.