Share extensions work much in the same way as action extensions, which are both part of extensibility. For example, if you want to share something to Pinterest from Safari, you no longer have to copy the URL and then open the Pinterest app. Instead, Pinterest and many other apps are now available via the Share sheet and can be accessed directly from Safari and many other App Store apps. That means you no longer have to go jumping around between apps ? All you have to do is set up share extensions.
Full story from the iMore Blog...
Facebook
Twitter
Instagram