As of writing, deviantART provides some APIs which third party developers can use in applications. This allows people to create applications such as Pickley's Sta.sh Client - applications that allow you to interact with deviantART away from the website itself, and even when away from your computer. A minimal OAuth2 API is exposed, which allows users to authorise applications with their accounts, which, in turn, allows applications to perform certain actions with your account.
The OAuth2 API could do with some changes, but is fine for the time being. Aside from OAuth2, deviantART provides an API for sta.sh, and some other basic APIs, which are documented on the developers page. While Sta.sh has a somewhat robust API, the rest of the website's features