Google Nexus 7 does not come with a back camera and as a result, there is no camera app on the device.
There is a front camera designed to be used for video calling purposes. But if you want to use it to take photos, a third party app is now available from MoDaCo.
MoDaCo has released Camera Launcher for Nexus 7 as a free download from the Google Play Store.
This very simple application adds a launcher icon for the Camera application. This allows you to use the Camera and Video Camera applications on your Nexus 7, including the funny camera effects. :)