what is the difference between elm327 and OBD-II UART? if we are using elm327 then do we need to use OBD-II UART also?

we are working on our final year project that is getting the current data from the vehicle and display it on an android phone using arduino and bluetooth  module nrf8001. currently we are working on obd II interfacing and our task is to get the data on arduino and only for  testing purpose  using lcd. we are confused of whether to use elm327 usb cable or obd-II UART for the intermediate circuit between arduino and car's obd-II. 
kindly help and if anyone can provide us with the arduino coding so it would be very helpful.

Either will work for you. Check the instructables listed here on the right for options and code.