What is the Object Relationship Overview in Salesforce?

In Salesforce, the object relationship overview links custom object records to standard object records in a related list. This is helpful to track product defects in related customer cases. Salesforce allows users to define different types of relationships by creating custom relationship fields on an object.

Master-Detail
Lookup
Many-to-Many
Master-Detail (1:n)

A parent-child relationship in which the master object controls certain behaviors of the detail object.

When a record of the master object is deleted, its related detail records are also deleted.
The Owner field on the detail object is not available and is automatically set to the owner of its associated master record. Custom objects on the detail side of a master-detail relationship cannot have sharing rules, manual sharing, or queues, as these require the Owner field.
The detail record inherits the sharing and security settings of its master record.
The master-detail relationship field is required on the page layout of the detail record.
By default, records can’t be reparented in master-detail relationships. Administrators can, however, allow child records in master-detail relationships on custom objects to be reparented to different parent records by selecting the Allow reparenting option in the master-detail relationship definition.
You can define master-detail relationships between custom objects or between a custom object and a standard object. However, the standard object cannot be on the detail side of a relationship with a custom object. In addition, you cannot create a master-detail relationship in which the User or Lead objects are the master.

Lookup Relationship (1:n)

This type of relationship links two objects together, but has no effect on deletion or security. Unlike master-detail fields, lookup fields are not automatically required. When you define a lookup relationship, data from one object can appear as a custom related list on page layouts for the other object. See the Salesforce online help for details.

Child row not automatically deleted when a parent row is deleted
No inherited sharing.
25 lookup relation relationships allowed per object.
Lookup field on child not necessarily required.
Many-to-Many

You can use master-detail relationships to model many-to-many relationships between any two objects. A many-to-many relationship allows each record of one object to be linked to multiple records from another object and vice versa.

To create a many-to-many relationship, simply create a custom junction object with two master-detail relationship fields, each linking to the objects you want to relate.