The medium defines the ways you can transfer images from the camera to the computer. You can either connect the camera directly to a computer using a cable or you can remove the card or disk from the camera and pop it into a special drive. After downloading the images, erase the camera's storage medium, and you're ready to take more pictures.
Serial connections are very slow and require special software to download the images. Serial cables came with all the older cameras. They are plugged into the printer or modem port of the computer.
USB (universal serial bus) is a faster replacement for serial. If you are planning on using USB, you can look at the outside of the camera box for the USB symbol. Just plug the camera in and out of the USB port while the computer and camera are turned on, and open a photo application like iPhoto to download and view your pictures.