Alcohol acts specifically in the central nervous system (spinal cord and brain) to depress signals. They do this through their actions on the excitatory (stimulating) and inhibitory (depressant) chemicals GABA and glutamate, respectively.  

At low doses it seems to work by activating the excitatory receptor (NMDA) and chemical (glutamate) in the brain and acting in areas related to memory, pleasure and thinking.

Higher dose seem to slow down (depress) the central nervous system by activating the inhibitory receptors related to the release of the chemical GABA. This leads to poor coordination, memory loss, blurred vision etc.

Very high doses can cause vomiting, coma and death through respiratory failure. The fatal dose is considered to be around 500mg of alcohol per 100ml of blood.