Ghosts were chosen from psychically-gifted individuals who were quarantined by the Terran Confederacy and trained from infancy to channel their psionic energies to augment their natural physical strength and endurance. Conscripts who successfully completed the Ghost Program's rigorous training and augmentations then served as commandos and assassins.
Ghosts are trained in target shooting, close-quarters combat (martial arts and possibly other techniques) and vehicular expertise. Ghosts are also trained to move quickly and are desensitized to combat; an example would be Sarah Kerrigan's training, who had a gun placed to her head by a guard and was ordered to kill the guard before he killed her. Kerrigan was subjected to this training at age twelve.
Ghosts were frequently tasked to locate targets for and provide targeting for tactical nuclear strikes. Their reconnaissance role was aided by cybernetic enhancements to their eyes.
Standard Confederate practice was to implant psychic dampeners into all ghosts to weaken and reshape their memories and as a safeguard, acting on the principle of neural resocialization; however these implants could be disabled.
I am the one, the shadow, the everlasting darkness.