A couple of schools (CBS, Tuck) have have had ED rounds that work somewhat like undergrad Early Action. But for most if not all other schools, the first round is simply an earlier submission and decision date.
At almost all schools, the third round is a much harder time to gain admissions than the first two rounds. So if you are planning on applying for matriculation in September 2005 you want to apply before the school's last round.
The question becomes trickier when you ask that same question about round 1 vs round 2. Here some will tell you that hotshots apply in round 1 and if you aren't a hotshot, you shouldn't apply in round 1. Personally, I disagree and I advocate Linda's rule: Apply as early as possible PROVIDED you don't compromise the quality of your application.
Never submit less than your best in a misguided rush to submit. At the same time, since hotshots apply at all times, if you are applying at the top schools, apply when they have the most spaces -- first round, assuming of course that you are followng Linda's rule.
Good luck!