Area2D

func _ready():
# [ ... rest of your code ... ]
    get_node("Area2D").connect("body_enter",self,"_on_Area2D_body_enter")
    get_node("Area2D").connect("body_exit",self,"_on_Area2D_body_exit")


func _on_Area2D_body_enter( body ):
    print("Entered Area2D with body ", body)
func _on_Area2D_body_exit( body ):
    print("Exited Area2D with body ", body)


KnematicBody

Movendo:

func _fixed_process(delta):
    var direction = Vector2(0,0)
    if ( Input.is_action_pressed("ui_up") ):
        direction += Vector2(0,-1)
    if ( Input.is_action_pressed("ui_down") ):
        direction += Vector2(0,1)
    if ( Input.is_action_pressed("ui_left") ):
        direction += Vector2(-1,0)
    if ( Input.is_action_pressed("ui_right") ):
        direction += Vector2(1,0)

    move( direction * speed * delta)

Colidindo:    
func _fixed_process(delta):
# [ ... previous code ... ]
    if is_colliding():  # colliding with Static, Kinematic, Rigid
        # do something
        print ("Collision with ", get_collider() )  # get_collider() returns CollisionObject

Continuando o movimento:

func _fixed_process( delta ):
...
# [ ... your code ... ]
    if is_colliding():
        var n = get_collision_normal()
        direction = n.slide( direction )
        move(direction*speed*delta)


RigidBody


func _ready():
	set_fixed_process(true)

func _fixed_process(delta):
	var bodies = get_colliding_bodies()
	#for body in bodies:
	#	print(body.get_name())