This catalogue of "HII regions" was compiled by Stewart Sharpless of the U.S. Naval Observatory in 1959 by examining plates from the Palomar Sky Survey plates in 1959. HII is ionised Hydrogen, i.e. Hydrogen that has lost its electron (HI is neutral Hydrogen). HII implies either high temperatures or some other form of energy to ionise the Hydrogen. Some of the more famous Messier and Caldwell objects are also in the Sharpless Catalogue, e.g. M42 & the Crescent Nebula. More information at Galaxy Map.
HII regions tend to emit strongly in H-Alpha light (fron the remaining neutral Hydrogen). This makes the Sharpless objects good targets when strong moonlight interferes with normal broadband imaging.