The type of cross you mentioned is sometimes also called the St. Georges Cross. There are many variations on the theme. The circle in the centre represents infinity. Circles and shapes derived from circles (such as the "fish" symbol": two bisecting circles) are a constant and recurring theme throughout Christianity, as well as many other faiths. They are symbolic of peace and the great cycle of life, death and rebirth. This particular type of cross is widely associated with Roman Catholicism.