Many popular games, like Angry Birds and Candy Crush, you can download to your smartphone for no charge. Thanks to a business model based on microtransactions, companies are able to keep the games free.
Microtransactions are small purchases made from within the app and typically run from a quarter to a couple of dollars each. They're easy enough for the casual user to ignore, but are very tempting for those who become dedicated to doing whatever it takes to excel in the game.
In-app purchases are also tempting to children who not only want to win, but tend to do it on their parent's dime.
In fact, there are some horror stories of children doing just that. In March 2013, a 5-year-old boy racked up $2,500 in in-app purchases within 10 minutes on his parents' iPad while playing Zombies vs. Ninjas. A few days ago a 10-year-old boy with learning disabilities charged his foster parents $670 while playing games on their Android tablet.
Luckily, there are ways to prevent these sorts of things from happening.