Seriously, they'll eat anything
One of the major aspects of Grey Goo that's likely to set it apart from other real-time strategy games is the inclusion of the titular Goo -- a race of nanites that can form into and break off from an amorphous blob. When they leave the motherblob, they become their own units, and go off to chainsmoke behind the 7-Eleven (or something like that).
If you're what we call a "visual learner," these screenshots might provide some context as to what the hell these things are. If you're the kinesthetic type, just go find some Play-Doh.