[Grandiagod]

Old Gods are different than Titans by quite a bit. Old Gods are "Gods" by the warcraft definition of immortality (not invulnerability, they just never die) and having worshipers. In fact the Lich King is the same genre of god C'thun is. (He's worshiped as a Death God by several races) The Old Gods are referred to by the Titans as parasites that corrupted Azeroth, but they ingrained themselves so tightly into the world that their removal would destroy Azeroth so the Titans locked them up underground when they reshaped Azeroth.

Titans are invulnerable to magic and physical attacks and to *most* magical weapons (I remember a case of where a legendary magic weapon hurt Sargeras once but I'm too lazy to Wowwiki it).