Apple says they've figured out how to implement third party multitasking without hurting performance or battery life, and they're demoing it now -- you just double click the home button and see a list of your apps, and you can just tap to switch between apps.
The Share button brings up a list of all the installed apps that can be used to share or exploit a photo -- for example, if you install Evernote on the camera then this button will give you the option of creating a note out of the photo you're looking at.