Chromebook ??

Help me out. I am having a difficult time understanding how a Chromebook differs from a tablet. I understand the obvious differences (physical keyboard & whichever OS), but other than that isn't it just a portal to the web and an engine for running web based applications. Isn't this exactly what all of these tablets are accomplishing as well? So why do you buy a Chromebook over say the Xoom. Because you want a keyboard and prefer Chrome OS over Android.