Zoology is the study of the life of animals. It is a diverse science and there are many different branches. Zoologists study all aspects of animal life from their cells to the animal itself.
Below is a list of branches of zoology and a brief description of each.
Parasitology - this is a very broad biological discipline, but it is basically the study of parasites and their hosts and how they relate to one another.
Palaeontology - the study of extinct and fossilized animal and plants.
Ethology - the study of animal behavior.
Acarology - The study of ticks and mites.
Arachnology - the study of arachnids and everything to do with arachnids. (Ticks and mites are also arachnids, but they have their own discipline - Acarology).
Cetology - the study of whales.
Entomology - the study of insects.
Herpetology - the study of amphibians and reptiles.
Ichthyology - the study of fishes.
Mammalogy - you guessed it. The study of mammals.
Neuroethology - the study of the natural behavior of animals. How they would behave in their natural habitat.
Ornithology - the study of birds.
Paleozoology - the study of fossilized animals.
Zoology is a branch of biology which is a very diverse field. The are numerous other disciplines of biology that are involved in zoology. Many overlap and studying in one area will certainly require some knowledge or expertise in another of the disciplines.