A (usually) hindering effect that is directed to the mind of the target. Feeblemind and Fear are two examples of a mind-affecting attack/state. Constructs, undead, and vermin are immune to mind-affecting spells. To determine the results of these effects, Will saving throws are usually used.
Bug: Targets that have the "immortal" flag set on them are immune to mind-affecting spells as well. So while trolls are not actually immune to mind-affecting spells, they will ignore any mind-affecting spell due to game engine limitations. The same is true for any characters set to immortal for plot reasons.