In 1 Sam. 21:15ff, they are giants with six digits. The author argues that the appearances in the poetic texts shouldn’t be conceptually identified. In Isaiah 14 they are identified with all the kings of the earth. They belong to the Netherworld, the Pit (vv. 9, 14). In Job 26:5 they are located beneath the waters under the earth.
The picture is somewhat different in the historical books, though not as much as the author suggests. Og is the last of the Rephaim (Dt. 3:11). They lost a battle in Gen. 14.
1 Enoch 6-14 attempts to tie these traditions together, noting that they are giants from the netherworld.
My own opinion is that they were a race of giants, an offshoot of one of the Nephilim clans. They were also royalty who have now been assigned to Sheol. There are hints in Ezek. 40-48 that they might be unleashed in the Apocalypse.