A taxonomist classifies and categorizes plants and animals. The profession is called taxonomy.
It can be a difficult job, because their task is to look at the similarities and differences in animals, and to determine which animals are related.
Sometimes, when the taxonomist recieves more information, he or she may change the category of an animal.
For example, the taxonomic classification of the Red Panda (Ailurus fulgens) has been very difficult. Is it related to the Giant Panda or is it related to the raccoon?