This article is from the Satellite TV FAQ, by Gary Bourgois email@example.com with numerous contributions by others.
A device containing a motor that is used to swing the satellite dish
to allow it to focus on individual satellites, which may be as close
to one another as 2 degrees of arc. Actuator arms are most commonly
found in 18- and 24-inch lengths (the longer the arm, the wider the arc
that the dish can "see".) In addition, a more expensive aiming mechanism,
called a "horizon-to-horizon" actuator, is capable of more precise aiming
(important on Ku band and closely-spaced satellites), as well as being
able to see the entire satellite arc from your location.
Many true dish heads have also added a second actuator to control
the VERTICAL aspect (elevation) of the dish, to enable them to
track inclined orbit satellites which "wobble" or vary in the vertical