Both stemming from research into finding the 'next gen' digital storage medium, two camps devloped similar but incompatible technologies as they have been unable to agree on a format standard. Given the way the lasers read the discs, HD uses a 0.6mm clear layer over the data (same as older style discs), whereas BR discs use only a 0.1mm clear layer. The thinner layer means more layers of data can be stored, but a higher manufacturing cost. That's it really. BR discs cost more but hold more. (theoretical storage figures are up to 200GB for Blu-Ray, and 60GB for HD DVD). That's how I understand it anyway...