A partner is a company with whom you have a business agreement, but who is not a customer or a vendor.
If you do not have partner records enabled in your account, an administrator can enable them at Setup > Company > Setup Tasks > Enable Features (Administrator). Click the CRM subtab. In the Partners section, check the Partner Relationship Management box, and then save.
You can associate promotion codes with partners on the Promotion Codes subtab. When customers have partners listed on their customer records, those customers can use only promotion codes associated with that partner.