Applying to a school with Early Action (EA) means you will receive an early response to your application. When applying EA, you will typically need to apply by November 1 or 15 and you will receive a response from the school no later than the end of December or beginning of January. For EA, you will have until May 1 to commit.
Applying to a school with Early Decision (ED) means that you are entering into a binding agreement. If you choose to apply with ED, you are telling the school that if you are accepted, you will commit to that school. ED applications are typically due by November 1 or 15 and sometimes, as late as January 1. If you are accepted ED, you must withdraw all outstanding applications.