An Application Programming Interface (API) is a set of tools that a system makes available so that unrelated systems or software have the ability to interact with each other. Typically, a developer uses an API when writing software that will interact with a closed, external software system. The external software system provides an API as a standard set of tools that all developers can use. Many popular social networking sites provide developers access to APIs to build tools to support those sites. The most obvious examples are Facebook and Twitter. Both have a robust API that provides developers with the ability to build plugins and work with data directly, without them being granted full access, as a general security precaution.

As you will see with this chapter, providing your own API is not only fairly simple, but it also empowers you to provide your users with access to your data. You also have the added peace of mind of knowing that you are in complete control over what level of access you can grant, what sets of data you can make read-only, and what data can be inserted and updated.