What's the difference between an Enrolled Agent and a CPA?

An Enrolled Agent is a federally recognized tax specialist.

Only enrolled agents are required to demonstrate to the IRS their continuing competence in all areas of taxation, representation and ethics before they are given unlimited representation rights before IRS. 

CPAs are state licensed and may or may not choose to specialize in taxes, while all enrolled agents specialize in taxation. Registered tax return preparers have passed a minimal competence test on tax forms for individuals, and have only limited representation rights. California CRTPs have passed a minimal California competence test.

NAEA Definition of Enrolled Agent