A Certified Public Accountant (CPA) or Enrolled Agent (EA), or as we like to refer to them, a Pro.

CPAs are individuals who've met the stringent requirements required by their state board of accountancy and passed the CPA Exam which is largely regarded as one of the more difficult professional exams with an average 50% pass rate. 

CPAs who are tax practitioners are subject to many different standards and ethics rules; including all enforceable tax ethics, Circular 230 of the Internal Revenue Code, state licensing boards and other regulatory agencies, professional associations, and various other laws and regulations.

EAs are also subject to IRS Circular 230 and must pass a three part exam that ensures they have the necessary knowledge to be able to prepare returns and represent clients before the IRS.  They too must maintain compliance with ethical standards set forth by the IRS as well as continuing education requirements.

Pros must remain up to date on their technical knowledge and licenses must be periodically renewed through continuing professional education.