HII regions are emission nebulae created when young, massive stars ionise nearby gas clouds with high-energy UV radiation. They are composed primarily of hydrogen and have temperatures of around 10,000 Kelvin.
They can extend over several hundred light years or be so compact that they do not even stretch 1 light year across. Correspondingly they have a large range of densities, from a few atoms / cm3, to millions of atoms / cm3 for the most compact regions.
In our Galaxy, HII regions follow a distribution pattern similar to that of the molecular clouds from which stars are formed, and are similarly concentrated in the spiral arms of other galaxies.