Spring Break definitely means more people and a bigger party atmosphere. I've cruised on both Spring Break and summer seasons, and I can tell you that the crowd was significantly larger over spring break. Now larger can be both good and bad, but in my case, it so happened that there were 20-30 of us partying in this one room the entire cruise (last cruise CCL Liberty Spring Break '08 )
Summer cruises have a more relaxed more intimate feel to them, and the parties, although still around, won't be as intense. Winter break is no different; I used to take these when I was back in New York, and they were the same as the summer ones. However, like I mentioned above, the real difference is between summer/winter break and spring break.