UMPC or Ultra Mobile Personal Computer, These are more portable than a laptop but contain all or much of the functionality. UMPC devices are expected to be small enough so they are easy to carry, deliver long battery life so you can access information for extended periods of time, have multiple wireless options so you can be connected anytime, and are location aware so it can adapt to your personal needs

I can't classify between UMPC and Sub-notebook. Sometime I've see some UMPC is Sub-notebook but some Sub-notebook is not UMPC then I notice UMPC is look like more Tablet PCs or PDA and they have built-in touchscreen display.

But I'm not interesting because they are high cost and unique (hard maintenance), some UMPC device such as ASUS R2H, Fujitsu U Series, Samsung Q1, Tablet PC, VIA Nanobook and GIGABYTE U60.

References
http://wiki.mobileread.com/wiki/UMPC
http://en.wikipedia.org/wiki/Umpc