The Preference Service is no longer an independent service. The implementation was deleted: https://chromium-review.googlesource.com/c/chromium/src/+/1928248
Some legacy implementation is still available here to support current usage. It has not been moved to be with the rest of the related preference code in components/prefs.
Preferences, also known as "prefs", are key-value pairs stored by Chrome. Examples include the settings in chrome://settings, all per-extension metadata, the list of plugins and so on. Individual prefs are keyed by a string and have a type. E.g., the "browser.enable_spellchecking" pref stores a boolean indicating whether spell-checking is enabled.
The pref service persists prefs to disk and communicates updates to prefs between services, including Chrome itself. There is a pref service instance per profile (prefs are persisted on a per-profile basis).
The original design doc is here:
https://docs.google.com/document/d/1JU8QUWxMEXWMqgkvFUumKSxr7Z-nfq0YvreSJTkMVmU/edit?usp=sharing