Folders in Apple’s iPhone OS 4 will allow to group apps in the Home Screen pages.
Creating a folder is as simple as dragging apps on top of each other to group them.

The feature names the folder according to its contents. The iPhone supports up to 11 pages on the home screen, or 180 items. This means that until now, you could keep up to 180 apps on the device. With folders for the theoretical total goes up to 2160 apps — with a maximum of 12 apps per folder.