Preferences (or short "Prefs") is Horde's name for user settings, i.e. any settings that can be changed by users individually. Preferences are available for almost all Horde applications and may be accessed by users clicking on the Options button in the top menu bar. There is also direct access to the individual applications' preference interfaces through the expandable Options menu entry in the left menu bar. Whether these buttons appear and under which condition can be configured by the administrator in Horde's configuration. Users can switch between the different applications' preferences by either using the entries in the left menu, or by picking an application in the top right drop down list inside the preference interface.
Preferences are organized in preference groups that are again divided into columns in the preference interface. All preferences, their descriptions, default values, groups, and columns are defined in the application's config/prefs.php file.
Preference values are cached in the user's session. This means that default value changes in the prefs.php configuration file don't have any effect until the user logs in the next time. Changes to the "cosmetic" settings that influence the interface like descriptions or grouping are applied immediately.
Preference values stored in the preference backend take precedence over the default settings defined in prefs.php, even if the preferences are locked. The idea is that you can override preferences for individual user by setting values for them in the backend. As a consequence, you have to delete existing values in backend, if you change the default value and want it to be applied to all users, even those that already changed that setting. This could be done with a SQL preference with the following query for example:
DELETE FROM horde_prefs WHERE pref_scope = '<application_name>' AND pref_name = '<preference_name>'
See CustomizingPreferences for a number of user-contributed preference settings and hooks.