Both of the big powerhouses in the digital SLR camera market -- Canon and Nikon -- make software with which you can control your camera from your PC through an attached USB cable. It is a nice tool for situations when manually triggering the shutter might cause shake (such as long exposures), or for automating multiple shots at timed intervals. But, naturally, the camera makers don't supply this software for free operating systems -- and just as naturally, the open source world has an answer. You can perform the same remote operation functions using gPhoto.