It's just a bunch of neurons wired together and they're either firing or not firing.
It's not just neurons; there are axions and a lot of other different cells. And I'm no neuroscientist but I'd bet that it isn't on/off; the signals are mostly chemical, meaning the strength of the signals between neurons can vary depending on the amount of chemical. IE, brains are analog, not binary.
I'm not a neuroscientist either, but here's what I picked up from Biology I:
Action potentials are all the same shape and size, so a neuron firing is digital in some sense. However, the information transmitted is generally analog (how much light is hitting your eye, how red something is, how much pressure / heat you feel). This information is conveyed through the frequency of the action potentials. It's sort of like frequency shift keying used in digital radio communications.