Aids are physiological responses to stimulus. Aids do not involve memory.

Cues are a signal for a specific learned behavior. It involves memory.

An aid would be more in the line of putting your calf on the horse's sides to raise his back--it's an automatic physical response.

A cue for a learned behavior, for example: wiggling your index finger for the horse to back up.

Our Icelandic Horses learn leg aids and cues very easily.