It’s a 256GB Solid State Drive (SSD). It’s going to run you $900 USD. For a comparison, the baseline unibody MacBook costs $1,299, so it’s pretty pricey.
The 256GB SSD option is available for both MacBook models as well as the 15-inch MacBook Pros. This option is not available on the plastic/white MacBook.
Prior to this week, if you were buying through Apple, a 256GB SSD was only available on the 17-inch MacBook PRos.