Sales forces *usually* have a closer relationship with the main company. Often, they're actual employees, but not always. In most cases, members of a sales force will actually take down the orders and transmit them to the main company, which is something most affiliates do not do. In some cases, members of sales forces get exclusive territories. Sometimes, members of sales forces get base salaries that are supplemented by commissions.
Affiliates usually have much simpler relationships - they direct interest and earn commission, subject to a few basic rules established by the company.