Hydrogen (atomic symbol H) is the simplest atom in the Universe. It consists of one proton and one electron. Hydrogen atoms are often referred to as HI, and when their electrons change spin orientation emit a radio photon with a radio frequency of 1420 MHz.
When ionized, Hydrogen loses its electron and becomes a single proton. This is referred to as HII. In the presence of massive stars, Hydrogen gas is ionised by ultra-violet radiation forming a massive sphere of ionised material known as a HII region or "Stromgren sphere".
Due to the orbits electrons like to populate Hydrogen atoms emit a series of spectral lines when the electrons move between orbits. Characteristic lines that Hydrogen emits are very prevalent in the Universe, and of great value to astronomers. These lines come in series, and have names like the Balmer series.