Android Folder Hierarchy On Android's root (/), what is the purpose of each folder? I want to learn the folder hierarchy structure. I would like to know the differences between Android 2.3 and 4.x, if they have different folders in "/".
First, you need to be aware of two facts:

Android uses more than one file system (think of &quot;multiple drives/partitions&quot; when comparing with your computer
while sharing a common base, directory structures might differ between manufacturers

So as starting points, I further recommend the file-system tag-wiki and the partition tag-wiki (you might also want to take a look at the most frequented questions using those tags).
In my answer, I will concentrate on the mentioned &quot;common base&quot;. However, there still might be deviations made by some manufacturers.
Partitions
As said, Android makes use of multiple partitions. In the file system, they are represented by &quot;directories&quot;, which serve as their mount-points:
┌─────────────┬───────────────────────────┐  
| Partition   | Explanation               |  
├─────────────┼───────────────────────────┤  
| /boot       | kernel &amp; Co.              |  
| /cache      | app cache                 |  
| /data       | user data partition¹      |  
| /data/data  | app data¹                 |  
| /dev        | devices²                  |  
| /mnt/asec   | encrypted apps (App2SD)   |  
| /mnt/emmc   | internal sdcard³          |  
| /mnt/sdcard | external sdcard³          |  
| /proc       | process information²      |  
| /recovery   | used in recovery mode     |  
| /system     | system ROM (read-only)    |  
└─────────────┴───────────────────────────┘

¹ Details below
² virtual file systems
³ these might differ. Often, /mnt/sdcard is the internal SD card, while the external SD card is found in /mnt/sdcard/external_sd.
Above list is far from being complete, but should hold the most important partitions.
Directories
Here I again will concentrate on the partitions which are most interesting (or this answer would get far too long and, for most readers, boring.
/data and /data/data
These are in most cases two separate partitions, but there might be cases where this is handled otherwise. One thing they have in common (add /cache here as well): they get wiped on a factory-reset, while the other partitions are usually left untouched by that.
As for the directories contained, I will again concentrate on a selection; most things here you either cannot touch without having your device rooted.
┌────────────────────┬──────────────────────────────────────────────┐  
| Directory          | Explanation                                  |  
├────────────────────┼──────────────────────────────────────────────┤  
| /data/anr          | traces from app crashes (App Not Responding) |  
| /data/app          | .apk files of apps installed by the user     |  
| /data/backup       | Googles Cloud-Backup stuff                   |  
| /data/dalvik-cache | optimized versions of installed apps¹        |  
| /data/data         | app data²                                    |  
| /data/local        | temporary files from e.g. Google Play³       |  
| /data/misc         | system configuration (WiFi, VPN, etc.)       |  
| /data/system       | more system related stuff (certs, battstat)  |  
| /data/tombstones   | more crash stuff (&quot;core dumps&quot;)              |  
└────────────────────┴──────────────────────────────────────────────┘

¹ for details on the Dalvik cache, see: dalvik
² each app gets its own data directory assigned here, using the app's package name. There might be a similar directory on your SD card, mostly used by apps with larger amounts of data.
³ usually, files are stored here temporarily to be installed/executed. Google Play e.g. downloads .apk files to this directory, before installing the downloaded app on your device

Of course, I intentionally skipped several directories. If you really want to go into details, this site might not be the best fitting place (as we deal with end-user questions -- and end-users normally don't touch these things). In that case, several places/resources might be recommended:

XDA Developers
good technical books on Android (e.g. Andrew Hoog's Android Forensics and Mobile Security, which goes much deeper into details here)
Where can I ask questions that aren't Android Enthusiast questions?