From BlenderWiki

[edit] Actuators

Actuators perform actions, such as move, create objects, play a sound. The actuators initiate their functions when they get a positive pulse from one (or more) of their controllers.

There are 18 actuators as of 2.49a:

Motion	Sets object into motion and/or rotation, there are different options from “teleporting” to physically push rotate objects.
Shape Action	Handles animations stored in shape keys and animated with shape actions.
Action	Handles armature actions, this is only visible if an armature is selected.
Constraint	Constraints are used to limit object’s locations, distance or rotation. These are useful for controlling the physics of the object in game.
Ipo	Controls Ipo animations, these can move, rotate, scale, change colour of objects and more.
Camera	Has options to follow objects smoothly, primarily for camera objects but any object can use this.
Sound	Used to play sounds in the game.
Property	Manipulates the object’s properties, like assigning, adding or copying.
Edit Object	Edits the object’s mesh, adds objects or destroys them, it can also change the mesh of an object (and soon also recreate the collision mesh).
Scene	Manage the scenes in your .blend file, these can be used as levels or for UI and background.
Random	Creates random values which can be stored in properties.
Message	Sends messages, which can be received by other objects to activate them.
CD	Plays CD music (might not make it to 2.5).
Game	Handles the entire game and can do things as restart, quit, load and save.
Visibility	Changes visibility of the object.
2D Filter	Filters for special effects like sepia colours or blur.
Parent	Can set a parent to the object, or unparent it.
State	Changes states of the object.

[edit] Subpages

1. 2D Filters
2. Action
3.    Common
4. Edit Object
5. IPO
6. Motion
7. Property
8. Scene
9. Shape Action