From NW itself:
"Why Early Decision?
If you are certain that Northwestern is where you want to enroll, we encourage you to apply under our Early Decision plan. Applicants who choose Early Decision send a strong, positive message to Northwestern. Given their high level of interest and overall academic and personal strength, Early Decision applicants experience a higher rate of admission.
As with all traditional Early Decision plans, you agree to withdraw all applications at other colleges and enroll at Northwestern if admitted. "
http://admissions.northwestern.edu/apply/index.html#tab-header