The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction. It is simply one kind of synapse. (The neuromuscular junction is also called the myoneural junction.) This video describes the anatomical, chemical and physiological responses at the NMJ

Also another quite awesome animation of the neurons, neurotransmitters, muscle fibers, actin and myosin in signaling the events from action potential firing to muscle contraction can be found here