Hi Chris,
A simple but fairly accurate summary would be that Walt Disney World is busiest when children are out of school and at the lowest crowd levels when children are in school.
The week between Christmas and New Year is historically very busy, along with Easter week and the July 4th weekend. After that, my experience is the next busiest times are mid-June through mid-August, Thanksgiving week, and Presidents week in February. Of course, any particular day may or may not be busy. I've been there during Christmas week when it's feels like the Magic Kingdom is overflowing with guests, and I've also been there on a day that week when it was fairly empty; perhaps everybody else was visiting another park that day. The weather can also impact the crowd levels.
My favorite times of the year to visit are January, February, May, October, November, and the first two weeks of December. The first two weeks of December are especially nice as the crowds are thin and all of the Christmas decorations are up; Mickey's Very Merry Christmas Party is also held in the Magic Kingdom during this time and it's a lot of fun for families. October is also a great time to visit as Epcot usually holds its
International Food and Wine Festival during that time and the Magic Kingdom offers Mickey's Not So Scary Halloween Party.
Write back if you have any more questions and have fun planning your first trip!