The Silhouette is a digital cutting tool for personal use. It connects to your computer just like a printer, but instead of printing designs it cuts them with a small blade. With the machine connected to your computer, you'll get access to all of the fonts on your PC plus the thousands of cuttable designs found on the internet.
Silhouette SD is the exactly the same machine as Craft ROBO CC330-20. Both are manufactured by Graphtec of Japan. The Silhouette brand is owned by Graphtec America whereas the the Craft ROBO brand is used for marketing in Japan.