Projectslibgphoto2 is the core of gphoto2 software. It is a portable library which gives access to literally hundreds of digital cameras.
gphoto2 is the command line interface to libgphoto2. It allows almost everything that libgphoto2 can do.
gtkam is the GTK2 graphical front-end to libgphoto2.
gphotofs allow you no mount your camera on Linux using FUSE.
gphoto 0.4.x is the old fashionned version of gphoto software. It is not longer being maintained.