There are many permissions that can be granted to apps, such as access to the camera, the microphone, your contacts (why does Google Maps need my contacts?), and your location. It’s a good idea to keep track of which permissions you’ve given to which apps, and revoke any permissions that are not strictly needed. For example, if you posted a photo to Twitter once, but you aren’t likely to do it again, it would be a good idea to remove the right to look at your photos from the Twitter app.
In Settings, scroll down and tap on Privacy. Here resides the master list of all permissions and which apps you’ve granted them to. Go through all of them periodically, and revoke any permissions that you don’t think a particular app needs.