Share Sheets are what Apple has named the popups you see when you share a photo, link, document, or any other sort of data with another app installed on your device.
After installing an app that allows for sharing, or use, within another app, you may have a hard time finding its icon on the sheet. The reason? Some apps require you to enable it before it's visible.
When viewing a share (or action) sheet, scroll to the far right until you find a button labeled More and tap on it. Next you'll see a list of all the services available for this specific sheet (for example, the sheet used in the screenshots is in the Photos app). You can then enable or disable apps, and rearrange them based on your personal preference. Unfortunately you cannot disable the stock iOS sharing options, even if you never use them.
You will have to do this for each type of document you intend on sharing. Meaning, Safari will have its own share sheet, documents (such as PDFs) will have its own, and so on. But once you get everything dialed in to your liking, you shouldn't have to fuss with it too much unless you're install a new app.
Prior to iOS 8, sharing data outside of stock iOS apps was very limited; some might even say it was crippled. With iOS 8, the problem is presumably solved, and as third-party developers begin releasing apps with sharing enabled, you're going to want to know how to better manage your share sheet.
For more helpful iOS 8 tips, be sure to check out our complete guide on how to use iOS 8.