Filesytem on Android I'm totally confused regarding Android file system and its relation to SD card.

While browsing my Asus Nexus 7 (2013) with file manager Astro from very root catalog I found directory sdcard. But there is also catalog /mnt/sdcard that points to the same location. Why do I need both of them?

I got mixed twice while browsing my rooted Samsung S4 Lollipop with external SD card. ES file Explorer. It has :

/sdcard/ with some information that is not on SD card. But why this directory is called SD scard?


Mount directory:

/mnt/
  /extSdCard/ with card information inside that can be visible on PC if I 
remove it from phone and put to computer.
  /sdcard/ again pseudo SD card info.


Totally confused:

/storage/
  /emulated/0/ that contains /sdcard/ info
  /legacy/ the same like /emulated/0/
  /extSdCard/ linked to real SD card
  /sdcard0/ again pseudo SD card info.


What is idea of such strange file system organisation?
There are already answers for all parts of your question, so let me sum up:


Android Folder Hierarchy has some general information
Confused by the many locations of the virtual /sdcard/ should answer the last part of your question
The /sdcard direcory and other directory conventions: how is the Android filesystem structured? might also be helpful


If you take a closer look at /sdcard from a terminal (or via adb shell) using the ls command, you will see it is a symbolic link (the Wikipedia article explains it in detail), usually created for at least two reasons:


make it easier for you to find it
have a "canonical place" for it, regardless of where it is physically


There's of course more to it, but that would become too lengthy for an answer here :)