let’s assume `/system` is the rom folder

| Partition | Explanation |
|———|—————————|
| /boot | kernel & Co. |
| /cache | app cache |
| /data | user data partition¹ |
| /data/data | app data¹ |
| /dev | devices, virtual fs |
| /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) |

`/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.

| 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 (“core dumps”) |
| /data/user | multi-user support, /data/user/0 is a symlink to /data/data|

Reference
——

[1] http://android.stackexchange.com/questions/46926/android-folder-hierarchy
[2] http://android.stackexchange.com/questions/48393/what-kind-of-data-is-stored-in-data-user-directory