In its strictest sense the bar is the court sitting in full term. There are many different national and state bar associations. Each state has it's own rules as to the requirements for membership. Anyone representing themselves as lawfully being an attorney or lawyer in a given state in violation of that states law on the unauthorized practice of law can be prosecuted administratively by the courts, civilly by individuals or criminally by the state as the law allows in the various states. In many states people trained in law that are not members of the bar are allowed upon application to the court to represent people in legal proceedings in federal and state administrative courts.
Examples are tax court, elder care, worker's compensation and others that various states allow.
The term lawyer is defined differently in different states. Esquire as used anywhere at this website is clearly intended to be an honorary addage referring to individuals that are ladies and gentlemen and that these people treat others in a similar way.