Facebook apps are applications that run on the Facebook platform so that you can easily use it using your Facebook account.
Facebook apps, also known as Facebook Applications, runs on the Facebook platform so that you can easily use the application with your Facebook account without having to create a new account for it. Such applications are located at apps.facebook.com, and you can grant special permissions to the application to access some of your data.
Such applications are usually developed by a third party vendor and hosted elsewhere. They use the Facebook platform to seamlessly get you integrated into their application, thus reducing the pain in getting a new account created to use the application.