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!
Commercial use requires 'releases' - editorial use doesn't. Stock photos can be sold for commercial (eg, marketing) or editorial (ie, journalistic) use. You'll have more opportunities to make cash if your photos are available for both, but photos containing people or property (including branding and logos) need signed releases to be sold commercially.
What does that mean for you? It means Nielsen will pay you $50 a year to keep their app on your favorite internet browsing device. The app itself collects statistics on your internet usage anonymously, so you never have to worry about any data being linked to you. And the best part is, the app takes up barely any space and doesn’t slow down your phone or tablet at all!