In this tutorial, we assume you're using an Arduino Uno, Arduino Duemilanove, Nano, or Diecimila. If you have another board, read the corresponding page in this getting started guide.
You also need a standard USB cable (A plug to B plug): the kind you would connect to a USB printer, for example. (For the Arduino Nano, you'll need an A to Mini-B cable instead.)
The text of the Arduino getting started guide is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the guide are released into the public domain.