Google Now is integrated with voice commands on Nexus 5. It is a nice little feature in which you can tell the phone what to do, through voice commands. Much like Siri from Apple. However voice commands on Nexus 5 won't work if you choose any language other than English (US) during the initial set up.
Most common mistake committed by Nexus 5 users is that they choose their regional language while setting up for the first time. Once you choose your regional language, the voice command 'Ok Google' won't work.
In order to activate voice commands on Nexus 5, you must change the default language to English (US). For this
Go to Settings > Language & Input > Language > English (United States).