WHAT ARE EXTENSIONS?
Introduced in iOS 8, extensions open up iOS in a way never done before. Whereas apps were previously tightly sandboxed from each other, extensions now lets them embed themselves into the OS and even work with each other while still remaining secure.
Right now, iOS 8 has six different kinds of extensions. Widgets on the Notification Center are one. The Share and Action sheet also gain extensions, as does photo editing in the Photos app. Storage extensions let apps open files from a variety of cloud storage providers, and keyboard extensions let you replace Apple’s keyboard with a third-party one.
Because developers need to code extensions for their apps, what’s available on your device depends on what apps you have and what its developers have done to support it.