Cookie stuffing involves placing an affiliate tracking cookie on a website visitor's computer without their knowledge, which will then generate revenue for the person doing the cookie stuffing. This not only generates fraudulent affiliate sales but also has the potential to overwrite other affiliates' cookies, essentially stealing their legitimately earned commissions.
I have been making money online for over a year now. Not in the big time yet, still consider myself to be learning my craft. I was doing it really ineffectively (using adsense and amazon along with a t shirt affiliate deal) and driving traffic almost exclusively though facebook. Probably averaged about $800 a month over the last year, mostly because Christmas brought an absolute ton of t shirt sales)
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.