The voltage depends on the total power of the unit.  Generally, units less than 15,000 BTU will use a 115 or 125 V circuit.  Generally, units over 15,000 BTU will use a 220V circuit.  To install a 220V model will likely require you to install some special electrical wiring or getting an electrician to do this for you.  When buying a window ac unit the buyer should verify that they will be able to accommodate the unit electrically.  The 5 most common window air conditioner electrical plug and receptacle types are listed below for quick reference.  The manufacturer will list the type of plug on the unit.  Check the plug type with the visual diagrams below and verify that the plug will work with your wall outlet.