Other forms: trainers
A trainer is someone who teaches or coaches someone, like the trainer at the gym who promises that she'll have you ready to run a marathon in four months.
Some trainers can teach you how to do sit ups, while others can instruct you to program a computer or be an effective leader. Dog trainers teach dogs to follow commands — and they teach dog owners to give commands effectively. The noun trainer comes from the verb train, "to discipline or teach," from an earlier sense, "to manipulate to bring a desired form," the way a gardener trains a rose to grow up a trellis.