Q:

Authentication concepts for Apps

I don't know whether i am asking a stupid question here.
I am just starting to learn and program android applications.
I wondered :

How is the mechanism of an application and its back-end server works?
How to have a user login and only the contents related to that user is shown?

Which is something  like when we login a Facebook account, we will have access to our information only. 
Had a looked at SQL server and Data store and it seems like the database is shared for everyone using the application. 

A:

Android supports client apps, AppEngine supports server apps, communication between them is HTTP(S).
Endpoints are libraries of client code generated from server projects.  Endpoints perform authentication by passing client credentials to the AppEngine server.  Client app developers use Endpoint libraries instead of developing networking software themselves.
One of the AppEngine services is the optional Users Service to look up information related to the client credentials.  If a server app needs to implement access control, it can be programmed to accept or reject requests per request detail and per user.  The application author is responsible for implementing such security.

