A registered agent (sometimes also referred to as "resident agent") is the person or an entity appointed to accept legal documents on behalf of the corporation. Legal entities, such as corporations and LLCs, must appoint a person or an entity to act as their registered agent, so in case someone sues them the papers will be served on the registered agent at the registered office.
There are no restrictions regarding who can be a registered agent other than the person must be located and available at a physical street address within the state during normal business hours.
The purpose of the Registered Agent is to maintain a consistent physical legal address in the state in which your company is incorporated or doing business. It also will receive governmental and legal correspondence, including your annual report notices, any delinquency notices and many other documents that your company may be required to respond or take action.
Most states require that a registered agent be listed for the business in the state of incorporation or a state in which it is doing business. The state will suspend any company if it fails to maintain an agent.
You or any other individual who meets the state requirements and who agrees to act as agent for the company may serve as Registered Agent. However, keep in mind the following disadvantages:
The Registered Agent's name and address are made part of the public record. You might not want to expose your private information*.
If you change your actual business address you will have to file a change of agent address with the state.
Because of those disadvantages it is advised to use a professional registered agent service, especially if your business is home based or retail.
* In a few states, e.g. Florida, Utah etc. the names and addresses of members/officers are made public.
REMEMBER: in order for your business to maintain good standing status it must also maintain a Registered Agent.