How do Custom Report Types differ from Standard Report Types?

Standard Report Types: Pre-built by Salesforce for standard objects and common relationships (e.g., Accounts with Contacts, Opportunities with Products). You cannot modify which objects or relationships they include. Custom Report Types (CRT): Created by admins to define which primary object, related objects (up to 4 levels), and fields are available in the report builder. You control whether related records must exist ("A must have B") or may exist ("A may or may not have B") — affecting how records are filtered. CRTs also allow adding fields from multiple related objects in one report layout, and pre-defining which fields appear by default. CRTs are the only way to report on certain relationships or include fields not available in standard report types.