webkit is the rendering engine used to render pages, vs the gecko engine that firefox uses, trident for IE, and presto for opera
so far webkit browsers seem to be faster than gecko but firefox seems to have something new coming out which can leverage the gpu to render pages quicker. which, if it makes its way into their mobile browser, and a lot of newer phones having GPU's, could make things interesting.
so being that a division of google makes this webkit based browser for android, it's mobile chrome in everything but name and officialness.