There are many type of reationship in SF, you can check this post to have a complete overview https://salesforce.stackexchange.com/questions/28624/relationship-between-account-and-contact
TLDR: salesforce standard objects have their own type of relationship
https://salesforce.stackexchange.com/questions/28624/relationship-between-account-and-contact
TLDR: salesforce standard objects have their own type of relationship