A rating agency is a private or public enterprise which carries out research in order to determine the creditworthiness of large companies and governments.
The difference between a rating agency and a credit bureau is that credit bureaus primarily measure the creditworthiness of individuals and small and medium-sized businesses.
Major rating agencies include Standard & Poor’s, Moody’s and Fitch. The credit ratings given to companies and governments by these rating agencies can have a major impact on investment in those companies or countries.
Each rating agency has its own method of assessing creditworthiness and uses its own rating system. You can compare the ratings used by major rating agencies here.