The Ichthys (Greek:ἰχθύς), also known as the "Jesus Fish", is a famous symbol usually associated with Christianity. Technically, it is the Greek word for "Fish". The Greek name of the fish (Iota Chi Theta Upsilon Sigma) is a backronym for "Jesus Christ, Son Of God, Saviour" (which, in Greek, is Ἰησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ).
Today, the sign of the fish can be seen on car bumpers as an open declaration that the owner of the car is a believer in Jesus Christ. Originally, the sign of the fish was a secret disclosure of ones faith in Christ in the context of State persecution of the followers of Jesus.
Signs were common long ago and were not only appreciated by the mind as a symbol, but it was understood to be something that you participated in or performed leading to allegience to the reality behind. That understanding is expressed by the various signs in the Old and New Testaments. Early Christians could be spared by offering a bit of incense to Caesar or have their lives forfeit by refusing to do so. From that time and shortly after, Christians would offer incense to Christ (or the book of the Gospels, itself a "sign" of Christ), as a way of affirming and expressing that Caesar is not Lord, but Christ is the Lord. Because of this mentality, it is doubtful that sharing the fish sign was merely a way of informing some one else that you were a Christian. It was an affirmation of faith in a time of distress and it bound you not only to another, but closer to Christ.
In recent years, the sign has been corrupted by "Darwin" fish. This is not to be confused with the original Ichthys symbol.