Other Stack Exchange users have already answered this question in posts elsewhere.

t0mm13b [writes](https://android.stackexchange.com/questions/43706/whats-the-best-way-to-restart-android-via-command-line-remotely#43708) 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 [another explanation](https://stackoverflow.com/questions/9598522/android-root-poweroff#9600769) of what Android does during a graceful shutdown:

>  *  It shuts down ActivityManager.  I think shutting down ActivityManager means that all activities will pass necessary lifecycle and, thus, the states of activities will be stored.  But I'm not sure.  I did not check.
>  *  Then, Android turns off the cellular radio interface.
>  *  After that, it turns off Bluetooth.
>  *  Finally, it tries to shut MountService down.

`adb reboot` is faster because it skips some or all of the above.