Roll-Up Summary

Quick answer: Use a master-detail relationship when the child record means nothing without its parent: it should share the parent's security, be deleted with it, and feed roll-up summary fields. Use a lookup when the child has a life of its own, with its own owner and sharing, and the link to the parent can be optional. Relationships are how Salesforce connects one object to another, like Accounts to Contacts or Orders to Order Items. The type you pick decides who can see the child records, what happens when a parent is deleted, and whether you get totals for free. Getting it wrong is expensive to fix later, so it pays to decide on purpose.