Some devices (like my Nexus 4) don't have an external slot for an SD Card. Instead, there's a root folder called
/sdcard/ that is treated the same way. But I also find other paths that all point to the same files -- there must be some kind redirects going on (I think this is called symlinks in Linux).
All these paths contain the same files:
/sdcard/ /storage/sdcard0/ /storage/emulated/0/ /storage/emulated/legacy/
Which path is the "actual" one? What about the other paths? What are they used for, why do they exist?