Great piece in this week's New Scientist on recent experiments which have actually shown these key braincells in operation in human brains (rather than inferred them from mu-wave diffusion or observed directly only in other primates).
In case you hadn't been paying attention, mirror neurons are braincells which act both as part of the motor and the cognitive systems - they fire both when an action is initiated and also when it is observed in another individual
They seem to be the key to us living the highly complex social lives that we do - they provide us with the means to interpret and predict the likely actions of those around us and thus negotiate public spaces and group life. (as well as being able to "feel" emotions at the flickering screens of a movie-theatre). They are believed to be the root of the (not exclusively human) empathy but also morality. And their malfunction seems to be behind the difficulty that autistic individuals' problems understanding the world (of other people) around them.
Go here to here the great "Ram" Ramachandran discuss neurons. Or here for something a bit geekier
Another piece of evidence to remind us just how brilliantly suited our brains are to the job of being the Super Social Ape and members of the Human Herd