Acetylcholine is a neurotransmitter while Acetylcholinesterase is an enzyme that inhibits its action.
Acetylcholine takes part in nervous co-ordination in transmission of nerve impulses. It acts as a neurotransmitter and conveys the nerve impulse through synaptic cleft i.e. the space between neurons. Acetylcholinesterase inhibits the neural transmission by breaking down acetylcholine.