The Cross of St. Peter (officially known as the Petrine Cross or colloquially Peter's Cross) is an inverted Latin cross. The origin of this symbol comes from the Catholic tradition that St. Peter was crucified upside down, as he felt he was unworthy to be crucified in the same manner that Christ died (upright). It is often used with two keys, symbolizing the keys of heaven.
The Alexandrian scholar Origen is the first to report that St. Peter was crucified head downward, for he had asked that he might suffer in this way. Some Catholics use this cross as a symbol of humility and unworthiness in comparison to Christ.

Read much more about the apostle Peter.