A reseller partner sells a vendor’s product directly to their client. In contrast to the relationship between affiliate/referral partners who are involved only at the start of the customer’s journey (it gets handed off to the vendor), a reseller partner owns the whole customer relationship.
Resellers are the most complicated partners to work with, and for good reason! They give you access to a relatively limited but highly qualified audience. This is why they often require more support and enablement resources.
Also see: “Affiliate partner”, “Referral partner”