It is basically a singing synthesizer application with a female voice, developed by Crypton Future Media.
Saying it just like that makes it sound kinda professional, but actually it is a really strong and wide marketing icon. I thought it was quite interesting when I made a quick search for its background in wikipedia, that the word "otaku" was not present at all. Actually I like to think of this character as a major otaku icon. It is present in anime, manga, music, video games, and, most surprising of all, it even appears in "live concerts".
Check one of its PV's below!
So, became a fan? ^-^