Why is there ice on my outside AC unit?

If you see ice on  the outside unit there is a problem, turn your system OFF.  First of all, understand, that it takes 3-4 hours for this ice to completely thaw.  The ice begins INSIDE your home in the evaporator coil, if you see ice outside, it means the evaporator coil is a solid block of ice.  There are five different things that cause your air conditioning system to ice up.  The most common problem, and one you can check yourself is a dirty air filter.  Check your filter, and if it's very dirty, replace it, but then leave the system off for at least 3-4 hours with your indoor fan running (turn thermostat switch to fan "ON" but leave system "OFF").  After this time, turn the system back on and let it run.  If it ices up again you need to call an air conditioning company to properly diagnose the system.  You may either be low on refrigerant or have problems with the indoor blower/evaporator coil.