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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 10,000 K. 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 include HI regions and molecular clouds. HI regions and HII regions are termed diffuse clouds and molecular clouds are termed dense clouds.
Nearly all the interstellar medium (ISM) is HII, but is generally much thinner (less dense) than HII regions; neutral atomic hydrogen regions (HI regions) form where this very diffuse material sufficiently cools. What are termed HII regions form after HI regions further cool into molecular clouds and hot, blue, early stars such as OB stars form. These produce ionizing radiation, ultraviolet with photons of sufficient energy to ionize nearby hydrogen in the molecular clouds, producing a more-distinct, higher-density region of HII. A Strömgren sphere is a model of such a region when formed around a single star. Small HII regions are known as compact HII regions or (smaller) ultracompact HII regions (UCHII) or (smallest) hypercompact HII regions (HCHII). Larger such 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 and constitute a sign that star formation has occurred recently. At the border between the HII region and the surrounding molecular cloud, the electromagnetic radiation (EMR) is insufficiently intensive to change the balance toward ionization, but sufficient to cause enough photodissociation to form an intervening photodissociation region. It is thought that the EMR from the bright star(s) and the heating of the gas pushes and compresses surrounding molecular gas, which can trigger nearby gravitational collapse and additional star formation in the surrounding 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.