Google Opinion Rewards is a survey app that you can earn credits to use in the google play store.
The first step is to download the app from the google play store. You will then sign in with your google account and answer a few demographic questions.
Once completed, you will receive, on average, one survey a week. A survey notification will appear when a survey is ready. The surveys are only a few questions and once completed your account will be credited. You can expect to receive a few cents to up to a dollar per survey.
The money that is earned can only be used in the Google Play Store. I have used the money for apps, movies and books, but there is much more. Personally, I like to check out the deals section in the books. I have found great books for under a dollar using money I received answering a few short questions. You should not expect to make a lot of money from this, but it is a fun way to get free media from the Play Store.