Apple's Find My iPhone service is a major asset to people who have lost their iPhones or have had them stolen. It uses the iPhone's built-in GPS to track the location of the phone and let the owner take actions like locking it so the person who has it now can't use it or deleting all the data on the phone.
You need to have set up Find My iPhone before your phone is stolen if you want to use the service. But what if you didn't install the Find My iPhone app on your device before it was lost or stolen? Does that mean you can't use Find My iPhone and your iPhone is gone for good?
I've got good news: even if you've never installed or used the Find My iPhone app (Download at iTunes), you still may be able to use the service to track your phone. This is possible because the Find My iPhone app isn't what makes your iPhone trackable.
Find My iPhone is a cloud-based service offered by Apple. This means the service lives on the Internet (not on your phone) and is accessible over the Internet. The Find My iPhone app is one way to use the service, but it's not the only one. You can also use it in virtually any modern web browser by just going to iCloud.com and logging in with your Apple ID.
So, just like your web browser is one way to use Find My iPhone, the app is simply another way to do the same thing. The app is not the service itself.
You can enable Find My iPhone on your phone even if you never install the app. If you've turned on iCloud on your device, there's a good chance that Find My iPhone was enabled. If that's the case, you're in luck--your device may be trackable.
So, you may be asking, if the app isn't required to use Find My iPhone, what's it for? The app is designed as a way to track your lost or stolen iPhone. You can do that tracking on a computer, too, but if you're trying to hunt down your device on the move, doing it from a friend or family member's phone where the app is installed is usually easier than using a laptop.