Usually, the ARES is organized by city and county and will be affiliated with the local municipality or county government.
The top of the command structure is the Section Emergency Coordinator, or SEC. This person is a member of the Section Managers Cabinet responsible for emergency communications.
The Section Emergency Coordinator has the District Emergency Coordinators or DECs working for them. Nominally a DEC will be affiliated with a county level or regional assignment.
Each DEC will have one or more Emergency Coordinators working for them. The Emergency Coordinator, or EC is the tactical commander within the organization. The EC is responsible for organizing a local ARES group, and serving as it's leader during actual operations.
Often an ARES group will also serve as a RACES group. To operate during a declared emergency, you would normally be part of RACES.