If you are going to build an android application (it can be any other mobile platform or web too) that manages all the user data on a central database, REST API will be good architectural option to do the communication between the app and the server.

If you consider Evernote, Wunderlist apps, these apps can uninstalled at anytime and once we install them back and login, all our data will be restored. This is because all the data will stored in a cloud database and communication b/w app and database will be done using a REST API.

This tutorial gives enough knowledge about building a REST API for very beginners. As this tutorial seems lengthy, I had divided it into 2 parts. In the 1st part we learn fundamental concepts of REST and do the required setup. In the 2nd part building actual API (writing PHP & MySQL code) is covered.