Definition
Creates a relationship that links this object to another object. The relationship field allows users to click on a lookup icon to select a value from a popup list. The other object is the source of the values in the list.
Lookup relationships are similar to master-detail relationships, except they don’t support sharing or roll-up summary fields. With a lookup relationship, you can link two different objects.
Example
The relationship between Account and Contact objects is a lookup relationship. A lookup relationship essentially links two objects together so that you can “look up” one object from the related items on another object.
Lookup relationships can be one-to-one or one-to-many. The Account to Contact relationship is one-to-many because a single account can have many related contacts.
Limits
You can create up to 40 lookup relationships for a single object.