Biology consist of many disciplines, and how you divide it depends on the starting point. If you start with organism, you can differentiate between microbiology (study of bacteria and other microorganisms), botany (study of plants) and zoology (study of animals). There are also subcategories like ornithology (birds), entomology (insects) or phycology (algae).
If you instead start with their natural habitats, you may talk about marine biology (everything that lives in the sea) or limnology (fresh water).
You may also use different scientific questions and methodologies as the starting point, like physiology & cell biology, ecology, evolutionary biology, systematics and conservation biology.