What do one of these do as I see them on ebay all the time for my bike

http://cgi.ebay.co.uk/REGULATOR-RECTIFI ... .m14.l1318

The regulator rectifier is a combined unit. It does the rectifying part as well the regulation part. It is part of a battery charging system. It usuallty gets an AC power feed from the stator coil of the generator (alternator). Most modern regulator rectifiers and Stators form a three phase system, so there are three wires coming from the stator feeding into the regulator rectifier. The regulator rectifier then rectifies the voltage; that is it turns the voltage from AC into an undulating DC. The voltage is then regulated; the voltage being limited to a maximum of about 14.5 volts and feeds this regulated DC out to the battery. There are some single phase, regulator rectifiers as well which perform a similar function but there are only two wires coming from the stator into the regulator rectifier which is then fed to the battery in the same way as the three phase system. Why are there two different systems? Three phase is more efficient and single phase are cheaper to produce.
Urm borrowed from the wemoto site