The file system in Android 4.2 and 4.3 is kind of obscure, IMHO. You'd think there'd be two top-level folders, one for the phone and one for the SD card. But no. Using the My Files app to explore the S4, the following seems to be the case.
/storage is the very top level.
/storage/emulated/0 is the top-level folder for the phone itself; beneath it are Download, Documents, DCIM (with pictures and videos, if the camera is putting them on the phone), folders created by programs, etc.
/storage/extSdCard is the top-level folder for the SD card is; beneath it also are Documents, DCIM (where the camera will store pics and videos if you tell it to put them on the card), and often folders created by programs (which can allow configurations, preferences, and data to be backed up easily from the card to a PC or other location).
If you connect the phone to a PC via USB cable, give it about a minute (during which you'll hear the standard Windows USB-device-connecting sound and later the USB-device-disconnecting sound). A new portable device appears in Explorer, under Computer, called (for my Verizon S4 still using the default device name) SCH-I545, and described as a Portable Media Player (at least on my Windows 7 PC). Click on this device in Explorer and it should show two Items, Card and Phone, and clicking on either of those will show a list of folders, the same lists as in My Files, but nothing about /storage.
I don't know the technical reasons why this is so, but that's the way it works on my phone. My Files will get you around on the phone once you learn where things are, but it's not an obvious system, and it sometimes fools apps as well. Ringtones need to go in a folder called Ringtones, but it seems that that folder can be on either the phone or the SD card.
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