What Is Difference Between Custom Report Types And Standard Report Types Salesforce?

Standard report types give you access to most Salesforce data.

Custom report types give you access to custom objects in Salesforce, or custom views of standard objects (like Opportunities), which your administrator configures.

Please go through:

https://help.salesforce.com/articleView?id=reports_builder_selecting_a_report_type.htm&type=5