The basic idea is to use a microbial fuel cell to breakdown organic matter and generate electrical power, but the hard part is making it efficient enough to allow useful activity. Bristol Robotics Laboratory has a couple of generations on this scheme now; the video shows the Ecobot II (from a story here).

It may be tempting to laugh at the poor performance, but when this machine's great-great-great-great.... child reads the comments, it will come after you, and your little dog too.