Grace Hopper was a Rear Admiral (lower half) (1 star) [1] where as Michelle Howard is promoted to Admiral (4 star) [2]. To put in perspective, ADM (now that her ceremony has occurred) Howard will be the No. 2 officer in the entire Navy. It is the first that a woman has held this high post in all of the US Armed Forces (previously, a woman was promoted to General in the Army, but did not occupy the No. 2 position).
[1]: http://en.m.wikipedia.org/wiki/Rear_admiral_(United_States)
[2]: http://en.m.wikipedia.org/wiki/Admiral_(United_States)