A petty officer is a non-commissioned officer in the navy in the United States and several other countries. He assists a warrant officer and his rank or position is similar to that of a sergeant in the army. There are different classes and grades of petty officers depending on the country and their naval system. A petty officer is also addressed by the abbreviated form of the word i.e. PO. Some of the other countries who have petty officers include Canada and the United Kingdom. Thus it is a marine lingo and is a Navy and coast Guard series of ranks. They have a badge or insignia which indicates the speciality and the rank of petty officer. There is a hierarchy among petty officers also. There is a senior petty officer, a chief petty officer and II or III class petty officers. All of them wear formal uniforms like the officers.