Periodically editing your Google account permissions is a smart way of ensuring your account remains secure. When doing so, you can view the apps you've granted permission to access your Google account (and your personal information).
You can always monitor and edit your Google account permissions from the website, or if you're an Android user, you can do it directly from your device.
You won't have to download an app to edit permissions, as it's included in the "Google Settings" app. This may be an app you've probably never even noticed on your device.
Launch your app drawer and navigate to the Google Settings icon. Once you've found it, select it. On the next screen, select the top option "Connected Apps."
You should then see a list of apps and services you've given permission to access your account. By tapping one of the apps, a new screen will pop up displaying the information the service has access to. At the bottom of the screen you'll find a "Disconnect" button. Should you decide you no longer want the service to have access to your account, tap it and confirm your decision.
In some instances you may find a small checkbox in the confirmation popup asking if you want to also delete all of your information from the service. If that's something you want to do, check the box before confirming the disconnect.
It's important to note you won't find any devices you've approved for access to your account -- only apps and online services. You will need to use the website mentioned above in order to edit devices.