They're name was given to them by the Luminoth, which in their language means "terror". Obviously they gave this name to the species for a reason - The Ing have dwindled the Luminoth's status to almost extinct, with only few being alive, a majority of the living ones frozen.
The Ing are feared by many, and have even taken down a Galactic Federation team with ease. Their emperor happens to have been the first of their kind, and was appropriately named Emperor Ing.
Because the species is not able to survive on the "light" Aether in their current form, they are required, if they wish to survive, to literally possess another being from the light world and take over it. Some species in the "light" world take a higher skilled Ing to take over it, though minor, easily killed ones can be possessed by almost all.
Once Samus Aran destroyed Dark Aether, all Ings were seemingly killed.
However, in Metroid Prime 3: Corruption, a type of Ing called Phaz-Ing, which as its name suggests has been (heavily) corrupted by phazon, appeared on the destroyed GFS Valhalla, where they could either be shot down or quickly destroyed by being ripped apart via Samus's grapple attack.
(Note: The following is the appearance of a standard Ing, as shown in the template above.")
A standard Ing is blackish blue and has five legs. It has only one red eye, which covers most of its head. On top of its rather small body are a few dangly things popping out, which while seemingly is hair, is more than likely not.