To contract a muscle, an impulse is transmitted from the nerve ending across a gap (a synapse), to the muscle fibre. The signal in this synapse is carried by a chemical called Acetylcholine, and it is this chemical that is blocked by Botox®. Hence the muscle does not receive the message to contract, and remains at rest.
 
The primary targets for Botox® in aesthetics are the very superficial muscles of the face and neck, the ones which attach to the skin, as it is when these muscles contract that we get wrinkling.
 
The deeper muscles of the face usually remain unaffected and so overall expressions are preserved.