1. By the mode of fertilization:
- Internal fertilization: The sperm fertilizes the egg inside the female's body. This is common in mammals, reptiles, birds, and some fish and invertebrates.
- External fertilization: The sperm fertilizes the egg outside the female's body, typically in water. This is common in fish, amphibians, and some invertebrates.
2. By the development of the offspring:
- Oviparous: The offspring develop inside eggs laid by the female. This is common in birds, reptiles, amphibians, fish, and some invertebrates.
- Viviparous: The offspring develop inside the female's body, receiving nourishment directly from her. This is common in mammals.
- Ovoviviparous: The offspring develop inside eggs that are retained within the mother's body until they are ready to hatch. This is common in some fish, reptiles, and insects.
It's important to note that these categories are not mutually exclusive. For example, a species can be both oviparous and internally fertilized, as in the case of birds.