Ichthys is the Greek word for fish, and the name of the symbol often called the Jesus fish. Unlike the cross, the Ichthys and the Cho-Rho are purely Christian symbols. The fish has several associations with Jesus, the founder of Christianity. The fish was thought to be a symbol of Jesus's compassion and giving. It is reminder of an event where he was said to have 5000 people with a few fish and couple of loaves of bread.
The Greek letters for the word Ichthys, pictured at below the Ichthys above, are also said to be a acronym for a phrase that roughly translates as "Jesus the anointed son of god, Savior". This symbol hearkens back to when Christianity was still a mystery religion and was persecuted by Rome. When someone would encounter another person they believed might be Christian, they would nonchalantly make a swish in the sand with a walking stick or foot while talking with them. If the symbol were unknown to the person they were talking to the swish would be ignored and no harm done, if they were in fact Christian it would be followed by swish of their own, creating the Ichthys, and both individuals would know that it would be safe to discuss religious matters, without becoming a meow meow treat.