https://trailhead.salesforce.com/data_modeling/objects_intro Overview of Objects Identify FieldForce.com automatically assigns an identity field (called ID) to every object, and manages the identity data in every record. This identity field usually comes in a 15-character case sensitive form, which you may have already seen while using the platform System FieldsCreatedDate - the Date and time wh..

https://help.salesforce.com/HTViewHelpDoc?id=relationships_considerations.htm Relationship LimitsEach custom object can have up to two master-detail relationships and many lookup relationships.Each relationship is included in the maximum number of custom fields allowed.Converting RelationshipsYou can convert a master-detail relationship to a lookup relationship as long as no roll-up summary fiel..