Social media marketing involves the use of social networks, consumer's online brand-related activities (COBRA) and electronic word of mouth (eWOM) to successfully advertise online. Social networks such as Facebook and Twitter provide advertisers with information about the likes and dislikes of their consumers. This technique is crucial, as it provides the businesses with a "target audience". With social networks, information relevant to the user's likes is available to businesses; who then advertise accordingly. Activities such as uploading a picture of your "new Converse sneakers to Facebook" is an example of a COBRA. Electronic recommendations and appraisals are a convenient manner to have a product promoted via "consumer-to-consumer interactions. An example of eWOM would be an online hotel review; the hotel company can have two possible outcomes based on their service. A good service would result in a positive review which gets the hotel free advertising via social media. However, a poor service will result in a negative consumer review which can potentially harm the company's reputation.
Affiliate marketing is the art of marketing products, services or information for others. It doesn't require that you ever house or warehouse a single thing. But it does require that you have an audience to market those things to online. Without that audience, whether it's through search engines like Google or social media channels like Facebook, you'll find a difficult time with affiliate marketing.