Relationships & Schema Builder
Objects become powerful when they connect. A relationship links records on one object to records on another, which is what lets you navigate from an account to all of its contacts and opportunities. Salesforce offers a few relationship types, and choosing correctly has lasting consequences for security, reporting, and deletion behaviour.
A lookup is a loose link. A Contact looks up to an Account, but each can exist independently — delete the account and the contact simply loses its reference. A master-detail is a tight parent-child link: the detail record cannot exist without its master, inherits the master's sharing and security, is deleted when the master is deleted, and can roll its values up to the parent with roll-up summary fields. A many-to-many relationship is built with a junction object in the middle that has two master-detail relationships, one to each side — for example a "Course Enrolment" object linking Students and Courses.
Because master-detail imposes strong constraints, a common design rule is: use lookup by default, and reach for master-detail only when you specifically need roll-up summaries, cascade delete, or inherited sharing.
Schema Builder (in Setup) gives you a visual, drag-and-drop canvas of your objects and their relationships. It is excellent for understanding an unfamiliar org and for sketching new structures — you can add objects and fields directly on the canvas. It is best for exploration and small changes; for careful, documented work most admins still use Object Manager.
Try this: create a custom object "Project", add a lookup from Project to Account, then open Schema Builder and watch the line connecting them appear. Add a second custom object and a junction between two objects to see a many-to-many take shape. Getting relationships right early prevents painful migrations later — it is one of the most consequential decisions an admin makes. Next we shape how users actually see these records with page layouts and record types.
