I believe they are the anthromorphized forms of a popular singing software. Basically anime girls based on songs who are attributed personalities by a burgeoning fan-base, despite the difficulties of using the actual software.
Hatsune Miku is the reigning queen of the Vocaloids, the long-haired, dark-aqua colored one with twin 'tails. Also synonymous with leeks for some reason, much like the infamous Loituma Girl meme, starring Inoue Orihime of Bleach fame.
http://en.wikipedia.org/wiki/Loituma...aloid_adaption
Fans of this particular 'franchise' have considerable power, there having been cases (or was that one case?) of fan-made Vocaloids being adopted as official by the original company. There also exists differing interpretations of the avatar's personalities there being some fan-on-fan inter-'community' 'violence.' This is understandable, as the Vocaloid's entire personal and emotional make-up are surmised by fans from the songs they sing, thus being heavily dependent on personal interpretation.
Don't know anything about it really.