ARES is the emergency communications service provided by the ARRL. It is part of the Field Organization and run by persons appointed by the Section Manager.
RACES is the emergency communications provided by the state or local Civil Defense, Civil Preparedness, Emergency Management or whatever it is called in a given area. It is run by persons appointed by the government (local or state) that it serves.
ARES can drill as much as they want and talk to all ham stations. RACES may only drill one hour per week and only may communicate with other RACES stations during a drill or an actual emergency.
ARES is activated before, during and after an emergency. Generally, ARES handles all emergency messages, including those between government emergency management officials. RACES, on the other hand, almost never starts before an emergency and is active only during the emergency and during the immediate aftermath if government emergency management offices need communications support. RACES is normally shut down shortly after the emergency has cleared.
Any amateur Radio operator can join ARES, as they don't need to be approved by the civil defense organization.
We always promote that ARES members join the RACES group whenever they can so in real case of emergency, they can contact other RACES stations without a restrictions. We also promote that RACES members join an ARES group, so they can have as mush training and drills as they wish.