When one of your customers makes a purchase, you purchase the product from a third-party company (the drop shipper, usually a manufacturer or wholesaler) for a lower price. This process is as simple as forwarding the order from your customer, a process that can actually be completely automated. (Remember you don't have any risk here of buying inventory because the sale has already been made).
The music industry might not be as strong as it was in the 80s, but there are still plenty of ways to make money online as a musician. Sites like SoundBetter let you sell your services as a songwriter, producer, or session musician to thousands of customers a month. While Musicbed, Music Vine, Marmoset, and SongFreedom are perfect for licensing your music to TV shows, movies, and web series.
While it often takes a significant amount of time and hard work to build up your affiliate income, if you have the ability to regularly publish high quality content that drives in targeted traffic to your website, this online revenue source can become quite substantial. Follow along with my experiment growing a new blog, called SmartWP about WordPress, into an additional revenue stream for my business.