Fertilization is actually a very complicated process there are many biologists who have spent their whole lives trying to understand the steps involved in fertilization! The process also varies in different living things; for example, plants do it a bit differently than humans. Even in the animal world, there is lots of variation for instance, some animals do it outside of the body while others do it inside (like us). For the sake of explanation, lets talk right now about fertilization in humans. Basically, fertilization occurs when a sperm (from a male) and an egg (from a female) join together to make a new living thing, a baby.

When Fertilization Does Not Occur:
Eggs are produced by human women about every month. But obviously women do not have babies every month - so almost all of these eggs do NOT get fertilized. The egg typically only has about one day to get fertilized before it dies. When fertilization does not occur, the dead egg is reabsorbed by the womans body, and the uterus that was waiting to nourish the egg sheds its lining (a process called menstruation) and starts over again in preparation for the next months egg to come along. Womens bodies will produce an egg every month like this until about the age of 50 or so.