CGI = Computer-Generated Imagery. It's the utilization of software designed to replicate everyday objects in the form of a photograph - either still photos or moving video. Today, photographers are able to utilize this technology to capture images that would be either too difficult or too expensive by camera alone. And best of all, the exponential acceleration of computing power is allowing image makers to more easily create realistic visuals in less time.
In fact, one day Hollywood stars will have digital representations of themselves that are as detailed as the real life versions. That means Brad Pitt won't have to take time out of his schedule to show up at a photo shoot, he'll just provide his CGI file that can be imported into the clothing and environment that GQ Magazine wants to showcase him in for their upcoming cover story.