If you have a blog or other type of site, you can build affiliate links to different services on the website. Many people use Amazon as an affiliate partner. For example, if you are a beauty blogger writing about different products, you can set up Amazon affiliate links on your blog so that whenever someone buys the product you mention on Amazon, you receive a percentage of the sale. Amazon is not the only affiliate partner out there. Here is good, in-depth information on affiliate marketing. 

I've now only got an SF rental condo and a Lake Tahoe vacation rental in my real-estate-rental portfolio. Although I miss my old house, I certainly don't miss paying $23,000 a year in property taxes and another mortgage, and dealing with leaks and managing terrible tenants. I drove by the other day and couldn't believe how much noisier and busier the street was than where I currently live. I wouldn't be comfortable raising my son there.
Two months into my work hiatus and I’m doing well. I’ve made a little bit more than I’ve spent and I’ve worked about four hours a day on average. Ultimately, I’ll know I’m ready to make the permanent jump to a 4 day work hour when my passive income plus side incomes equal my day job income. Until then, I’m going to go back to my day job and keep grinding it out.
The owner of this website, Eric, is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking http://www.my4hrworkweek.com to Amazon properties including, but not limited to, amazon.com, endless.com, myhabit.com, smallparts.com, or amazonwireless.com.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.