From Bulbapedia, the community-driven Pokémon encyclopedia.
A type specialist is a Pokémon Trainer who specializes in just one elemental type. These Trainers are most commonly Gym Leaders or members of the Elite Four. There are also some Trainer classes that specialize in one type, such as the Bug Catchers who specialize solely in training and battling with Bug-type Pokémon
Some notable trainers would include Brock, the Pewter City Gym Leader who specializes in only Rock-type Pokémon, and Misty, the Cerulean City Gym Leader who specializes in only Water-type Pokémon.