A Pokemon Trainer is some person who captures Pokemon using a doohickey called a Pokeball. All they do with Pokemon is train them so they can be better at battling dumb Oddish. Pokemon Trainers also enter them in contests so they can get pins with fancy textures.
It is unknown why people became Pokemon Trainers. However, they just train Pokemon so they can face the evil Gym Leaders to get a Badge. After they collect all eight badges, they can go and battle the Elite Four, the evil organization who rules the leaders, and the champion, the evilest of 'em all. Another reason why it is invented is because Pokemon Trainers actually gamble when battling.