If the recent news about the 5-year-old who managed to spend $2,500 in 15 minutes on in-app purchases on an iPad has you a little concerned: don't worry, in-app purchases are incredibly easy to turn off so no one can accidentally spend money when you don't want them to.
Turning off in-app purchases is simple:
Head into Settings > General > Restrictions, and hit "Enable Restrictions.
Enter your passcode and scroll down to "Allowed Content."
Toggle "In-App Purchases" to "off."
If you want, you can also change the password requirement to "immediately" so that when you hand your phone over no purchases happen accidentally. Disabling in-app purchases at least gets rid of the most annoying (and easy to miss) purchases. Android users don't always have this option, but the few apps that do have in-app purchases should require you to enter your password to pay for them.