Custom Report: Custom report types give you access to custom objects in Salesforce, or custom views of standard objects (like Opportunities), which your administrator configures. For example, your administrator can create a custom report type that gives access to Opportunities, plus related fields from Products.
Joined Report: A joined report can contain data from multiple standards or custom report types. You can add report types to a joined report if they have relationships with the same object or objects. ... A joined report consists of up to five report blocks, which you add to the report to create multiple views of your data.
Custom Report: Custom report types give you access to custom objects in Salesforce, or custom views of standard objects (like Opportunities), which your administrator configures. For example, your administrator can create a custom report type that gives access to Opportunities, plus related fields from Products.
Joined Report: A joined report can contain data from multiple standards or custom report types. You can add report types to a joined report if they have relationships with the same object or objects. ... A joined report consists of up to five report blocks, which you add to the report to create multiple views of your data.
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The below article has complete information about Joined Reports and Custom Report Types
https://help.salesforce.com/articleView?id=reports_defining_report_types.htm&type=5
https://help.salesforce.com/articleView?id=reports_joined_format_concepts.htm&type=5
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