A creature of spider-shape, of the spawn of Ungoliant, who dwelt in the mountains on the western borders of Mordor.
Notes
1
The earliest definite date we have for Shelob is c. II 1000, because we know that she occupied Cirith Ungol 'before the first stone of Barad-dûr' which was laid at about that time (from The Two Towers IV 9 Shelob's Lair). We also know, from the same source, that she originally came there 'flying from ruin' - this is indefinite, but ties in neatly with the War of Wrath a thousand years earlier. The suggestion here is that she originally occupied Beleriand, probably in Ered Gorgoroth, and fled southwards to escape its destruction.
2
If 'Shelob' were an Elvish name, the 'e' sound would be short, giving a pronunciation something like 'shehlob'. Her name, though, is a compound of two English words, 'She-lob', so we have assumed a standard English pronunciation.