You can find affiliate marketing offers on sites like ClickBank, CJ.com and Rakuten LinkShare, amongst many others. Search for the right offer and ensure that you present it to the right audience and don't spam people about it. Do your marketing ethically. If you're looking for more information on effective affiliate marketing, check out Pat Flynn's Smart Passive Income website.
What’s the catch? None, really. Cash back apps act as affiliates for many online merchants, which means that whenever you make a purchase through one of the apps, they get a small commission — but then, they give you a portion of that commission as “cash back”. For example, if I buy a pair of Nike shoes through the Ebates app (or website) and spend $75, Ebates may get a $10 commission but then they’ll pass $7 back to me. It’s basically a way to get sale prices on stuff that isn’t on sale!