The enlisted rating badge for Petty Officer Third Class and above consists of two parts. The chevrons indicate pay grade (rate). Between the chevrons and the eagle is an insignia indicating the Sailor's job speciality (rating). Insignia are white on blue uniforms and Navy blue on white uniforms. Sailors in pay grades E-4 through E-6 can be addressed as "Petty Officer (name)" or [as example] "Boatswain's Mate Third Class (name)". The latter would be written as BM3.
Chief Petty Officers are always referred to as "Chief", "Senior Chief", or "Master Chief" as appropriate. As example: "Chief Jones" or in subsequent references, just "Chief".
The linked tables show the rating insignia for various Navy specialities.
Information Courtesy of U.S. Navy