What are the letters after doctors names?

Post-nominal letters, also called post-nominal initials, post-nominal titles or designatory letters, are letters placed after the name of a person to indicate that the individual holds a position, educational degree, accreditation, office, or honour. An individual may use several different sets of post-nominal letters. The order in which these are listed after a name is based on the order of precedence and category of the order. Post-nominal letters are one of the main types of name suffix.