A zoologist is a scientist whose subject is animals. Not a vet, and not a zoo keeper — though the words look close, and students mix them up all the time. A vet treats sick animals. A zoo keeper feeds them and cleans their homes. A zoologist studies them: how their bodies work, how they behave, where they live, how one kind of animal is related to another, why a bird sings the song it sings. A zoologist may work in a laboratory, or in a university teaching and writing, or outside in a forest for months watching one group of monkeys. Some zoologists do work at a zoo, which is where the confusion starts, but the job is the studying, not the caring.

The word is neutral and quite formal — it is a job title, the kind of word you meet in a science book, a news report, or a TV nature programme ('zoologist Dr Chen says…'). People do not usually say it about themselves in a friendly chat; they would say 'I study animals' or name the animal: 'I work on frogs.' In everyday talk, English speakers often stretch it and call anyone who knows a lot about animals a zoologist, even without the degree — that is loose but common, and nobody objects.