The Classic UI is the desktop-oriented user interface that as been available since CQ5 (the predecessory to AEM). The following documentation on authoring is focused on this UI.
The touch-enabled UI is the new standard user interface for AEM, designed for use on both touch and desktop devices. Please see the standard authoring documentation for further information.
Because the touch-based UI is the standard UI of AEM, classic UI is no longer maintained. This means that there is certain functionality that is not available in the classic UI.
Regardless of the choice of UI, there are certain concepts and ideas common to authoring, which are well-described here in the standard authoring documentation.