Android OS have /data partition which contains user-generated data in folders like /data/app and /data/data, but /data/system folder is most important in device functionality. It stores system configuration files to be used by hardware or controllers and other data which is not accessible by applications. This folder contains system configuration files required for maintaining device's state. Access to this directory is limited to the system apps only, and its content cannot be viewed, if the device is not rooted.
Following is the list of files placed in the /data/system directory with their brief description. The list is not complete and also may vary depending on the Android OS version.
appops.xml: It is configuration file used by the AppOps service which controls application permissions.
called_pre_boots.dat: This file is used by ActivityManager to store pre-boot broadcast receivers.
batterystats.bin: It is binary used by the BatteryStats service to keeps power statistics of different application installed in the device.
gesture.key: It stores data about lockscreen patterns created by user.
inputmethod: This directory store IME configuration and contains subtypes.xml with IME locales, keyboard layout sets etc.
locksettings.db: It is SQLite database containing the lock screen settings for the device.
netpolicy.xml: It is configuration file used by the NetworkPolicyManagerService.
netstats: This directory is used to store NetworkStatsService statistics.
packages.list: This file is used by the PackageManager service containing the list of all packages or apks installed in the system.
packages.xml: This file is used by the PackageManager service to store metadata of all installed packages, like UIDs and requested permissions.
profiles.xml: This configuration file is used store metadata of device profiles including sound mode, airplane mode and vibration etc.
password.key: This file stores password hash.
usagestats: It is directory which store files for the UsageStats service. It contains usage-history.xml which has info of all applications and components with the last time a user used them.
users: This directory stores information about multi-user support. It contains userlist.xml which has information of all users in the system and other data like widgets, wallpapers and themes.