Basic Structure

Boot image and kernel required to start device
/boot

Contains the entire Android OS 
/system

Recovery image to recover device with software failure
/recovery

Temporary files used by the OS and Apps
/cache

Storage of miscellaneous system settings
/misc

Emulated external storage accessed by the system and user
/sdcard

Optional partition to provide additional storage space
/sd-ext
                  

General

Application databases
/data/data/(package)/databases

Shared Preferences
/data/data/(package)/shared_prefs/

Applications installed by user
/data/app

Pre-installed applications
/system/app

Encrypted Apllications - App2SD
/mmt/asec

Internal SD Card
/mmt/emmc

Internal / External SD Card
/mmt/sdcard

External SD Card
/mmt/sdcard/external_sd