Affiliate marketing:

Affiliate marketing? I'll explain. S.O.P.S. is affiliate marketing. Just that. One merchant sells any commodity. I'm physical, digital, a membership site, or a subscription. Affiliates offer merchant products for a commission.
Example: You can join TARGET's affiliate program if you have a website. Signing up for their affiliate network, or any affiliate program, will give you product links for your website. TARGET will earn you a commission if a website visitor hits your link and makes a purchase. Simple.
Merchants have affiliate programs. Macy's, JC Penny, Amazon.com, Old Navy, Best Buy, and many sell actual things.
ClickBank.com sells digital items.
With ClickBank, you may promote hundreds of products and make a commission for each sale.