almost all heli's have a gyro, but some come with seperate gyro's and HH gyro's. Basically a HH gyro holds your tail in one spot, and a rate gyro dampens the movement of the tail. That's so that humans with slower reflexes than a gnat can actually fly the heli =)

check out this video for some more info
http://www.youtube.com/watch?v=_TLD8CqsN0M