it is using android 2.2 froyo, another operating system for mobile devices, with its own libraries, on a linux kernel...
http://en.wikipedia.org/wiki/Android_(operating_system)
The whole point of "rooting the device" is this : android is secured, meaning the end user doesn't have superuser rights (administrator in windows language), and cannot alter system files, so cannot customize his device at will (as Linux allows the user to do). Rooting solves the issue and lets you install what you want, or even use the device for other means than Sony intended to.
for people interested, read this translation of the original russian thread where the rooting method was explained :
http://translate.google.com/translat...hp%3Ft%3D21458