For Christians, a fish is perhaps most famous as being the earliest known symbol of Christ. The letters in the Greek word Ichthus were believed by Christians to represent the first letters of the words, "Iesu Christos Theou Uios Soter" - or "Jesus Christ God's Son Saviour."
There are numerous mentions of fish in the Bible. For example, Jesus was referred to as being a "fisher of men", and his act of feeding the hungry five thousand people was achieved using five loaves and two fishes. Today it's still an icon steeped in symbolism and is used by followers of Christianity to signify their beliefs and celebrate Christ. As such, fish symbols are often seen decorating the back of cars, or worn as badges or pendants.
In other parts of the world, fish are regarded as symbols of water - the element in which they live - and representatives of life and fertility. This is due to their reputation as being powerfully reproductive and for laying huge numbers of eggs. In China, they're also seen as a symbol of plenty and good luck.