Other Stack Exchange users have already answered this question in posts elsewhere.
t0mm13b writes that, while displaying your carrier's shutdown animation:
- Android is safely shutting down vital parts of the runtime.
- The OS is also broadcasting intents to tell apps and services to gracefully close. These, in turn, flush their caches of all data and shared preferences, save what-nots to the sqlite database, et cetera.
In other words, apps and services are given a chance to do their cleanup systematically.
[Commands such as
adb reboot] are harsher. They actually bypass the safety mechanisms for a graceful shutdown.
Elsewhere, Yury offers a different explanation of what Android does during a graceful shutdown.