Google has implemented a server-side upgrade to their Search app for Android that allows you to start the camera app using voice commands. On Google's own Nexus 5 you can launch the Search app by saying "OK Google" from the home screen. On all other Android devices (version 4.1 or higher) you have to be in the Google Now interface to do the same.
You can then "take a photo" to open the camera app in stills mode or "take a video" to open it in video mode. For stills mode, the commands "capture a photo" or "capture a picture" can be used alternatively. Once the app has opened, however, you still need to hit the shutter or video button to take an image or record a video.
Voice commands in Android are not new of course. You can do all sorts of things speaking to your phone, from navigating to an address to setting an alarm. The camera commands are a welcome addition though and might help capturing that decisive moment. You can find a full list of all Android voice commands on Google's support site.