A Credit Bureau is an agency that research, determine and collect credit information from various sources [such as data furnishers, which can be creditors, debtors, debt collection agencies or offices with public records (court records that are available to the public)] about the financial market and then sells them for a fee to creditors (a person who lends money to someone, or whom a debt is owed).
What Credit Bureau basically does is that they look into an individual financial capacity; his previous borrowing habits, how he pays back, etc. and then give the information to a lending institution so that they can decide whether or not to lend such a person.
Therefore, they basically help the creditors have relevant information in lending out money and making basic financial decisions. However, they are not responsible for deciding who the individual pays credit to or what portion comes to them; they only collect information about that individual credit score and give the information to lending institution (institutions that lends out money).
Clients who go to credit bureaus for credit information a includes; banks, mortgage lenders, credit card companies and other financing companies. Credit bureaus also supply information on the interest rate of debtors on their loan (how much an individual or company will pay as interest in addition to the original loan that is borrowed).
It is to note that the Credit Bureau are not gods, therefore, they are prone to make mistakes or inaccuracies in their report about an individual.