What is the main difference between the XBOX 360 core system and XBOX 360?

The product description I have is, XBOX 360 core system delivers the same powerful gaming as XBOX 360 but starts with the basics. Okay, what are the basics and does it make a big deal in the gaming experience?

the core system cost $300, look like a normal system, no hard drive, wire controller, and can only play xbox 360 games.

the premium system cost $400, 20 gb hard drive, play some original xbox games, xbox live, downloadable arcade games, wireless controller, and other.

the elite system cost $480, 120 gb hard drive, hdmi, and the same as the premium.

My Xbox 360 Collection (Read Description)