Receptor: There are different types of receptors within the peripheral nervous system. There can be free nerve endings for pain and other types of receptors within the layers of the skin for sensations such as touch and pressure. These receptors take imformation from a stimulus (such as pain) down a neurone to the central nervous system (brain and spinal cord). Here the stimulus is made sense of and the sensory neurone from the receptor, synapses with a motor neurone (this neurone ultimately ends up at the effector). Effector: There are different types of effectors within the body such as those resulting in movement (i.e. Muscles) or those resulting in substance secretion (i.e. Glands). As above the receptor brings sensory information to the central nervous system and the motor neurone takes this information to the allocated effector group (such as a muscle) and causes an action.
Putting them both together: stimulus of stepping on a pin on the floor - pain receptors receive this information and send it down a sensory neurone to the central nervous system - here the information is made sense of - the sensory neurone synapses with the allocated motor neurone for that particular action - the information goes down the motor neurone to reach the effector, which in this case is a muscle group - the impulse causes the muscle to contract and withdraw your foot from the pin.
So to clarify: the function of a receptor is to receive sensory information, the function of an effector is to produce an action in response to that information from a receptor. Examples being a pain receptor in the skin and a muscle group being an effector.