Have lots and lots of apps? Folders are a great way to declutter your iPhone's home screens. To make a folder you just tap on an icon until it starts to jiggle, and then you drop that icon on top of another. The folder is named automatically with the type of applications you are grouping, but you can also rename each folder with any label you'd like (and change it again later, if you wish). Keep in mind, however, that you're limited to 12 applications per folder.
If an app within a folder has a notification to display, a badge shows up in the top-right corner of the folder to alert you. To open a folder, just tap on it; repeat that action to close it.