If you have used iOS for any period of time, you have seen the share sheet.

When you click on the share icon, you are presented a default list of apps and actions that can be performed. This is the default share sheet. In previous versions of iOS, we were limited to the default apps and actions that Apple built into the software. Now with iOS8, any app can add the ability to be featured in the share sheet from any app installed on your device.

Here is how you add the apps and/or actions you use the most.

Each of these screens will let you show/hide what you want and reorder the way they appear in the scrolling list.
The best way to get a good handle on how this works is to tinker a little.
Go to the photos app, click on a photo, hit the share button, and do an initial setup.
Happy Sharing!