A taxonomist is a biologist that groups organisms into categories. A plant taxonomist for example, might study the origins and relationships between different types of roses while an insect taxonomist might focus on the relationships between different types of beetles.
A taxonomist is not to be confused with a taxidermist, a person who specializes in the stuffing and mounting of dead animals. It's true, both words share their origins in the Greek taxis meaning "arrangement." But in the case of a taxonomist, it's not dead animals that are being arranged, its hierarchies of species.