Even though humans are animals, the term "animal study" refers to studies using non-human animals as subjects. Animals are used in studies when there would be potential harm from performing a study with humans.

Animals can be used in invasive studies and "sacrificed" so tissue samples can be taken at the end of the study. Therefore, we can gain information from animal studies that would be impossible in human studies. (know any humans willing to let someone take a sample of brain tissue?)

The usefulness of an animal study depends on the ability to find a suitable "animal model". Humans have a lot of similarities to, say, rats (some more so than others!), but rats are not humans. So animal researchers need to find animals that respond in a similar manner to humans regarding what they are studying. For example, vitamin C is a required nutrient for humans, but not rats, so rats are not a suitable animal model to use for vitamin C studies.

You have to be cautious when making generalizations from animal studies to the human population.

For example, very high doses of saccharine have been shown to cause cancer in laboratory animals.

However, the amounts of saccharine consumed by the rats were much more that a human could realistically consume. In addition, the types of tumors rats develop are often types not found in humans. So it is not really proper to say that saccharine causes cancer in humans.

Animal researchers must follow very strict guidelines to ensure the humane treatment of animals, yet some people do have ethical concerns about using animals for research.