Question: What is UID on Android?

Is it ID of a particular user or of a particular application?

Answer: 
If you're referring to [this UID](http://developer.android.com/reference/android/os/Process.html#myUid%28%29): Android is based on Linux, so basically it's the same UID you have on a Unix-like OS. When installing an app / package, Android by default[1] creates a UID specifically for that package, so that it can have its private resources / storage space. When no packages are using that UID any more (which could be shared), the UID is deleted.

1. You can override this behavior with [android:sharedUserId](http://developer.android.com/guide/topics/manifest/manifest-element.html#uid), but it has some [drawbacks](https://stackoverflow.com/questions/5529846/androidprocess-and-process-name/5530160#5530160).
This ID is particular to the application

Android assigns a unique user ID (UID) to each Android application and runs it in its own process. Android uses this UID to set up a kernel-level Application Sandbox.

Refer to this doc <https://source.android.com/security/app-sandbox>