The XBox 360 comes in two configurations - the core system and the pro system. The core system is the more basic of the two, and does NOT have a hard drive. Those people who have never had an XBox before might wonder why you need a hard drive. Really, it is critical to be able to download new content, store large save game files, and store music / photos.
The XBox 360 Core System is sort of like the XBox 360 "Lite". It has just the very bare basics you need to play. You get the base XBox 360 console unit, one wired controller, and standard AV cables to connect the unit to your TV. That's it. This is FAR less than you get with the advanced set. For example, the advanced set has a wireLESS controller, which I highly highly recommend. I hate wires. Also the advanced set has a high definition cable, giving you far higher quality on many TVs.
Of course the actual UNIT itself is exactly the same in both cases. There is only one XBox 360 console. It has a detachable faceplate, so you can customize it with cool designs. It's just under 8 pounds in weight, and 3.3 by 12.2 by 10.2 inches. For speed, the XBox 360 has 3 separate cores that are each 3.2 GHz in speed. That's faster than many peoples' home computer systems!
The built in DVD player is 12x dual layer, making the XBox 360 great to serve as your home entertainment system.
Note that you can NOT play any old XBox games at all if you don't have a hard drive. Even if you choose to get this Core unit for money reasons, I really suggest you buy the hard drive as an add-on!
XBox 360 Launch Information