It's a Garmin product, they publish two "North America" versions (Which don't include Mexico and points south). NT and non-NT. NT has fewer, larger, segments and is takes up less space on a mSDHC card. (about 1.1 Gb for the US, Canada, Hawaii, and Puerto Rico.)
You don't say what GPSr you are using, and some older units don't take NT, so check to make sure yours will. Otherwise, get the non-NT version, which will load smaller chunks.
The DVD version will be locked to a single GPSr. The mSDHC version isn't locked, but you won't be able to use it to plan trips on your PC, so I'd suggest going with the DVD version. If you don't have a PC, but use a MAC, I can't help you, but you should be able to find something on Garmin's site.