As the name implies the BD stands for Blu-ray, and ASRock has packed a full blu-ray drive into their ION330HT machine. That is the only difference here between the ION330-HT machine. Here is the spec sheet:

Processor Atom 330 (Dual Core)
Memory 2 GB (2x1GB) (4GB Max)
Hard Drive 320GB 5400RPM SATA
Video Ports 1 VGA, 1 HDMI
Other Ports 6 USB, 1 eSata, 5 jack Audio Port, 1 Optical SPDIF Out
Operating System None
Wifi 802.11b/g/n
Optical Drive Blu-ray Combo Drive
Included Accessories Media Center Remote

Prices vary, but are around $550 and available at:
Amazon:

Newegg:

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