Brushes

OK, all kidding aside now, there are two basic types of large generators of AC power; Synchronous and Induction. The difference is in how the generator rotors (rotating portion) produce magnetic fields so that the windings in the stator (stationary portion) can produce power (you can get permanent magnet generators for small amounts of power, but they are too expensive for much over 1kW). I will not go into AC theory, if you don't know that, you have to start off waaaaaay below our level here.

An induction generator produces electricity by virtue of the rotor being excited by the stator magnetic fields, through what might be described as "mutual induction"; the stator magnetic fields are created by existing AC power and their expansion and retraction cuts the conductor bars in the rotor to induce current flow, which then creates a magnetic field that interacts with the fields in the stator to make it spin. Once excited, the rotor can be driven FASTER than the rotating magnetic fields of the stator windings and the energy will flow the other direction. But you may have noticed that I said that the stator uses the EXISTING AC power to provide the excitation. That means that you cannot produce electricity with an induction generator unless you are ALREADY hooked up to an existing grid to supply that excitation energy.

So if you want to produce electricity from a generator that is all by itself and connected to nothing when it starts (called "island mode"), then there must be some means of putting electricity onto the rotor so that it becomes an electromagnet that can then interact with the stator to produce electricity. The brushes are how that energy is transferred from whatever your source of excitation power is, i.e. a battery or a small DC generator, to the rotating portion, the rotor. The "commutator" is the part on the rotor that the brushes connect to.