This circular symbol is an ancient Christian symbol. Within the circle are the greek letters ΙΧΘΥΣ (Ichthys)meaning Fish, and the individual letters standing for:
Ἰησοῦς Χριστὸς Θεοῦ Υἱὸς Σωτήρ
Iēsous Christos Theou Huios Sōtēr
"Jesus Christ, Son of God, Saviour."
Howard [ Just Imagine ] 22-Jan-2008