Schema Builder for objects and relationships
Key Takeaway
Schema Builder is one canvas for viewing and changing your data model. What you create there is a real part of your org, but moving objects around changes only the diagram.
Schema Builder is a visual tool in Setup that shows your org's objects, fields, and relationships as a diagram on one canvas. It is a single place to view and change the objects and relationships in your app. You can design, build, and modify your data model (schema) there without moving between Setup pages. Schema Builder is enabled by default and available in all editions, in both Salesforce Classic and Lightning Experience.
What Schema Builder shows
Schema Builder shows fields and relationships for both standard and custom objects. It displays details such as field values, required fields, and the lookup and master-detail relationships that connect objects.
Because everything is on one canvas, you don't have to click from page to page to see how objects relate. Schema Builder is useful for understanding complex data models and for showing a co-worker how data flows through your system.