The DVDs you buy aren't "recorded", they're pressed. A normal CD has one pressed layer. A normal DVD also has one pressed layer. The dual-sided DVDs you buy today are two layers bonded back to back and hold 10 gigabytes.

A dual-layered DVD is two layers bonded one behind the other. It holds 9 gigabytes. To read each layer, the DVD player changes the focus of it's laser. That's the short "pause" or delay you experience when playing any dual-layered movies.

A dual-sided, dual-layered disc would have four layers of data and would hold up to 18 gigabytes. Essentially, it's two dual-layered discs bonded back to back (though I haven't actually seen one yet). Up until now, the technology required to produce those discs at satisfactory levels didn't exist. As far as I know, The Stand will be the first DVD-18 disc.