From a recent Education Week article: "Popularity of Early Decision Up, But Not Its Edge":
Early Decision (ED) is a process where students make a binding commitment to their first-choice school if admitted. Some students apply ED in hopes of receiving an advantage with a higher admissions rate. The article notes that there is a slight advantage:
Applying ED to a school is a big decision, and the wise student should know how much of an edge s/he is getting in exchange for giving up flexibility and options regarding scholarships and financial aid.