Alarm events are used to sequence things to the number of updates or 'steps' the game has processed.
Step
Are for determining what happens each time the game updates.
Movement
Collision
Collision events occur when objects come in contact with each other.
Physics
Are when events trigger in the Box2D physics engine.
Input
Keyboard
Are for handling keyboard input.
Key Press
For handling when keyboard keys are pressed.
Key Release
For handling when keyboard keys are released.
Mouse
Are for handling mouse input.
Joystick
Are for handling joystick input.
Gamepad
Are for handling controller/gamepad input.
Graphics
Draw
Occur after the end step event and are for when you are ready to draw objects to the screen.
Draw GUI
Is for drawing things on a layer on top of everything else rendered, basically buttons and things. The event is called after all regular draw events occur and performs depth sorting on the objects and resets the projection to orthographic ignoring scaling and other view transformations.
Other
Other
Events not covered in other categories, such as those defined by the user.
Room
Are for when things happen to the room, such as it restarts or ends.