Question: What is UID on Android?

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

Answer: 
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>
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).