There are three types of fish reproduction. The first and most common one, found in 97% of fish, is ovaparity. The other two types are oviviparity and viviparity. As in humans, male fish produce sperm, and female fish produce eggs in their ovaries.
The most common fish reproduction, ovaparity, is an external fertilization process in which unfertilized eggs are laid. This is called spawning. A few species of fish, such as salmon, migrate long distances to spawn. Depending on the species of fish, eggs are laid in various places and methods to protect the eggs from predators. Some species prepare nests, some make bubble structures to protect the eggs and others incubate the eggs in their mouths. The percentage of eggs laid externally that survive to become fish is very small, so multiple eggs are laid at the same time.
Viviparity is an internal fertilization and development process most familiar to humans. An embryo is attached to the mother's blood supply, similar to a placenta, so it receives direct nourishment from the mother and is born alive.
Oviviparity is also an internal fertilization and development process, but the egg is not connected to the mother's blood supply. The embryo is developed inside the egg, which provides nourishment. This fish is also born alive.