All fish reproduce sexually through a process called spawning. Like all sexual reproduction, it involves the union of sperm from a male fertilizing the egg of a female.
Though the entire reproductive process is sexual, many times the fertilization of the eggs of the female take place outside of the body rather than internally as happens with humans and other mammals.
Female fish of most species lay a large number of eggs in an area. The male of the species then produces sperm in a substance known as milt. The milt is then released from its body as it passes a patch of eggs.