A report type defines the set of records and fields available to a report based on the relationships between a primary object and its related objects. Reports display only records that meet the criteria defined in the report type. Salesforce provides a set of pre-defined standard report types; administrators can create custom report types as well.
For example, an administrator can create a report type that shows only job applications that have an associated resume; applications without resumes won't show up in reports using that type. An administrator can also show records that may have related records—for example, applications with or without resumes. In this case, all applications, whether or not they have resumes, are available to reports using that type.
There is a limit to the total number of custom report types you can create.