Ultrabooks are
typically at the thinnest, lightest, and smallest end of the laptop spectrum. Think the MacBook Air. However, they also
typically are not as powerful as thicker laptops, and ship with SSDs instead of full HDDs (i.e. faster boot times, but significantly less storage space). Also, and most notably, all ultrabooks do not have a CD/DVD drive--this shaves off a ton of weight and space at some minor inconvenience.
http://en.wikipedia.org/wiki/Ultrabook
The best ultrabook I know of out there at quite a fair price: ASUS Zenbook UX32VD. 1080p display, light and thin (it's an ultrabook after all), really good build (that doesn't just try to mimic the Air), average battery life, SSD+HDD hybrid drive gives you the best of both worlds, powerful processor (although it's Ivy Bridge, not Haswell) and it comes with a discrete GPU. And it goes for about $1100.