For example, the example in symfony2:

Ens\JobeetBundle\Entity\CategoryAffiliate:
  type: entity
  table: category_affiliate
  id:
    id:
      type: integer
      generator: { strategy: AUTO }
  manyToOne:
    category:
      targetEntity: Category
      inversedBy: category_affiliates
      joinColumn:
        name: category_id
        referencedColumnName: id
    affiliate:
      targetEntity: Affiliate
      inversedBy: category_affiliates
      joinColumn:
        name: affiliate_id
        referencedColumnName: id

A more detailed explanation such as the generator option, and the documentation for manyToOne and OneToMany options
I am not very clear about the doctrine official website