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).
In Android, `UID` is actually called `AID`, which is used to identify the owner of a process and the owner of a resource. Binding those two together, 
it becomes the backbone of Android application sandboxing mechanism.

A full and detailed description of how it works and what does it means for application need a long article, which [you can find it here](http://pierrchen.blogspot.com.au/2016/09/an-walk-through-of-android-uidgid-based.html).