Of course, we're mostly concerned with things that are on the screen, but there are also things you can hint at that aren't on the screen.

The screen is the user's viewport into your app and it forms a mental model of how they see your interface. By showing the edges of items, it's possible to show the user that there's more to be seen just off the screen.

This technique, when used well, can provide a subtle visual cue to the user that there are more items to be found, while at the same time not taking up too much screen space.

In both of these examples, this isn't the only way to get to these other sections—there's a related video view in YouTube already—but it does provide a nice hint for new users and a shortcut for experienced ones.