I am familiar with Ubuntu 14.04 and once its installation is complete I can install the NVIDIA graphic card driver from the Software & Updates. But when I installed Elementary OS Freya and then tried to install the Nvidia driver I couldn't find it. Why is that?
Because it is somewhat hidden in elementary OS (as it doesn't have it's own Software Center).
Ubuntu Software Center -> Edit (at the top menubar) -> Software Sources -> Additional Drivers.
I know it's a pretty weird place for a thing that is so important but I think we have to wait for Loki and automatic hardware detection.