One of the new features of iOS 8 is Extensions, a way to add cross-app functionality to iOS. We have seen some extensions already; when opening a PDF in Mobile Safari, you get an "Open In…" option which shows the list of apps you have installed that you can use to read a PDF.
In iOS 8, this is expanding in a variety of ways. Right now, Extensions fall into the following categories: Actions, Custom Keyboards, Document Providers, Photo Editing, Sharing, and Today. This makes it easier to do certain things on iOS, like open a document stored in Dropbox, make some changes, and then email it off to someone else. Sharing extensions are particularly useful in iOS 8, making it so you can do a variety of things with a webpage in Mobile Safari.
Here's how to turn one on:
First, launch mobile Safari, and when you're on the desired page, tap the Share button:
Share sheet appearing in Safari in iOS 8.
Swipe the list of options all the way to the left, and you'll see a 'More…' option.
Share Sheet swiped right with the More button showing.
From there, you can enable or disable extensions and even adjust the order of the list, so the ones you use most are the ones displayed:
Sharing extensions list in Safari in iOS 8.
Enable the ones you want to use, and when you go back to the Share sheet, voila!
Hey look! It's Transmit! Just like I wanted!
For Safari, that's all you have to do! Extensions aren't automatically enabled, so keep in mind that when you install a new app with this feature, you will need to go turn on the extension in Safari. If you want to learn more about the technical aspects of Extensions, Ars Technica has an in-depth writeup.