Both are types of college applications that allow students to get an admissions decision early. Students can only apply to one school using Early Decision (ED). If they are accepted, they are then committed to attending that school. Even if they receive offers from other schools, they cannot accept them. The advantage of ED is that students are more likely to receive an offer than by doing a standard application.
Early Action (EA) allows students to receive their admissions decision, but then still have the flexibility to wait until “Decision Day” (usually May 1st) to commit to a school. Students may apply to as many EA schools as they want, unless they are applying to a school that has implemented Restrictive Early Action (REA). This policy—did we really need an even more complicated application process?—asks that students refrain from applying to any private schools in the early round.
The list of schools employing REA is very small, including only Georgetown University, Harvard University, Princeton University, Stanford University, the University of Notre Dame, and Yale University. An applicant to any of these schools may, as an example, also submit an early application to Michigan or Virginia, but not to Tufts or Boston College.