A credit bureau is also called a credit-reporting agency. A credit bureau is in the business of gathering, maintaining and selling information about consumers' credit histories. It's all they do.
A credit bureau will collect information about your payment habits from all kinds of sources. The credit bureau stores this information in a computer database. Then, it sells this information to companies and lenders in the form of credit reports.
When you apply for a mortgage, the lender will order your credit report from at least one credit bureau, but usually all three. The lender does this in case one credit bureau had something reported to it and it wasn't reported to the other bureaus. The tri-merged report is the result of merging the three credit reports from the three bureaus.
Note that the credit bureau charges the mortgage lender a fee for every credit report requested. This is how the credit bureau makes money.
No credit-reporting agency will recommend whether or not to give you a home loan. It is up to the mortgage lender to evaluate your credit report and then decide whether or not to offer you credit. The three leading credit reporting agencies in the US are: Trans Union.