Difference between /mnt/sdcard and /sdcard I'm in the beginning of Android kernel study and I'm looking at every directory in rooted Android. 

I realized that the same directories were in those two: /mnt/sdcard and /sdcard. What's the difference between them?

(The directories that I found were:

Alarms,Android, Audio, Books, DCIM, Download, Images, LOST.DIR, Movies, Music, Notifications, Pictures, Podcasts, Ringtones, Video)

I'm using Android 4.1.2.
The directory /mnt houses the mount points of different Android partitions. (hence the name mnt) Actually the "real" directory is /mnt/sdcard. /sdcard is a Symlink (short for Symbolic Link) to that directory, intended for easy of usability. (That is also explained  here Are files created in "/storage/emulated/0" supposed to appear in "/storage/sdcard0"? and the answer by Izzy on that question is quite useful and detailed) It is possible to access /mnt/sdcard by ADB or a well designed file manager. /sdcard directory serves as a way for normal apps/users to interact with that data.

You can read more about Symlinks here. This article (also the source of some facts I stated above) may be useful for a basic understanding about the Android filesystem. (It is/may be outdated for current Android versions, but you are using an old Android version, so it may still be useful for you)