Now, it is not a just another IDL(Interface Description Language: used to define an application interface). It is way less complicated. Protocol Buffers are way simplier.
Protocol Buffer is an innovative way to store the data. By using it, one can define data structures(DS). These DS are then compiled to produce code to parse.
Google neither uses XML nor does it stores data in raw-form. The google has gone one step ahead and has made it avaliable to the open-source community too!