The Igigi are hardly ever discussed on their own. They are discussed against the Anunna, or Anunnaki, and the relation between the two names is one of the open questions of the field. Nicole Brisch, writing for the Oracc project Ancient Mesopotamian Gods and Goddesses, says outright that the term Igigu is as complicated as Anunna, that it is not entirely clear what distinguishes one group from the other, and that the question needs a comprehensive new study.
What can be said is that the two words behave differently at different dates. Igigu is first attested in Old Babylonian texts and occurs only in Akkadian contexts; its Sumerian logographic equivalent, nun-gal-e-ne, means the great princes. In Atrahasis the Igigi are plainly the junior party, working for the Anunnaki. From the Middle Babylonian period onward the pair is read the other way about, with the Igigi as gods of heaven and the Anunnaki as gods of the underworld. In some texts the two names are used as though they meant the same thing, and in others, such as the poem of Erra, they are held apart.
The size of the group is contested as well. Von Soden argued for seven, Kienast for eight, and Black and Green give ten, each on different evidence; Kienast proposed Ishtar, Asarluhi, Ninurta, Nuska and Shamash among the members. Some gods appear to belong to both groups at once. The etymology is no better settled: Old Akkadian, Amorite and Arabic origins have all been argued for. No cult place for the Igigi is known, and one position holds that they belong to literature alone rather than to worship.