There are many different kinds of generators that could be used in a wind turbine, right now I am going to just group them in three different types.
An induction generator is a type of electrical generator that is mechanically and electrically similar to an induction motor. Induction generators produce electrical power when their shaft is rotated faster than the synchronous frequency of the equivalent induction motor. Induction generators are often used in wind turbines and some micro hydro installations. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or commutator.
Induction generators are not self-exciting, meaning they require an external supply to produce a rotating magnetic flux, the power required for this is called reactive current. The external supply can be supplied from the electrical grid or from the generator itself, once it starts producing power or can you can use a capacitor bank to supply it. The rotating magnetic flux from the stator induces currents in the rotor, which also produces a magnetic field. If the rotor turns slower than the rate of the rotating flux, the machine acts like an induction motor. If the rotor is turned faster, it acts like a generator, producing power at the synchronous frequency. In the United States it would be 60hz.
The common down side of using an induction generator in a wind turbine is gearing. Typically you need an induction motors to run 1500+ RPM to meet the synchronous so a gearing is almost always needed.
Permanent Magnet Alternators
Permanent magnets alternators (PMA) have one set of electromagnets and one set of permanent magnets. Typically the permanent magnets will be mounted on the rotor with the electromagnets on the stator. Permanent magnet motor and generator technology has advance greatly in the past few years with the creation of rare earth magnets (neodymium, samarium-cobalt, and alnico). Generally the coils will be wired in a standard three phase wye or delta.
Permanent magnet alternators are can be very efficient, in the range of 60%-95%, typically around 70% though. As a generator they do not require a controller as a typical three phase motor would need. It is easy to rectify the power from them and charge a battery bank or use with a grid tie.
It is easy to build a permanent magnet alternator, even for beginners. This is a common choice for home builders. I will have some great information on this site a little later that will take you through the design and building process. You just need to understand a little science and have some sort of mechanical competency.
Note: Car alternators are not PMA but actually have a field coil instead of permanent magnets, and are typically very inefficient around 50%. They typically need to be spun 1500+RPM to get any real power out of them, but with a belt or gear arrangement can still do a decent job.
Brushed DC Motor
Brushed DC Motors are commonly used for home built wind turbines. They are backwards from a permanent magnet generator. On a brushed motor, the electromagnets spin on the rotor with the power coming out of what is known as a commutator. This does cause a rectifying effecting outputting lumpy DC, but this is not an efficient way to “rectify” the power from the windings, it is used because it’s the only way to get the power out of the rotor. A good brushed motor can reach a good efficiency, but are typically at most 70%.
There are many great advantages to using a brushed motor. One of the biggest reasons is because typically you can find one not requiring any gearing and still get a battery charging voltage in light wind. They are also quite easy to find, they can be purchased from eBay, surplus supply stores, industrial supply stores, and can find them on different things that might get thrown away or given away (like a treadmill).