What is a social letter of introduction?

A social letter of introduction was especially important in the 18th and 19th centuries. It was used in formal situations to introduce one's credentials. For example, an ambassador would write a letter of introduction to let others know of all of his accomplishments. It was an important part of social interaction. This also played an important part in social classes. Often one would not socialize with a certain group of people without proper introduction. This could have been done with a letter, in person, or via third party (telling one person, who tells another).