Acetylcholinesterase (AChE) is an enzyme that breaks down acetylcholine (ACh), a neurotransmitter, into acetic acid and choline. AChE is found in the synaptic cleft, pre- and post-synaptic nerve terminals, and red blood cell membranes: 

    Function
    AChE's primary role is to stop neuronal transmission and signaling between synapses. It prevents ACh from spreading and activating nearby receptors. 

Location
AChE is mainly found at postsynaptic neuromuscular junctions, especially in muscles and nerves. It's also found on red blood cell membranes, where it's the Yt blood group antigen. 
Inhibitors
Organophosphates, which are a component of pesticides and nerve agents, inhibit AChE. 
Therapeutic uses
AChE inhibitors are used to treat a variety of conditions, including: 

    Myasthenia gravis: A neuromuscular disease that causes muscle weakness. AChE inhibitors were first used to treat this disease in 1932. 

Glaucoma: A disease that damages optic nerves. AChE inhibitors are used to protect retinal ganglion cells from ocular hypertension. 
Alzheimer's disease: A common form of dementia. AChE inhibitors are used to increase ACh activity and moderate AD symptoms.