Hi!
For rules about what exactly the differences are check out this site:
http://en2.wikipedia.org/wiki/American_and_British_English_Differences
basically its this:
-re becomes -er: center, theater
-our becomes -or: favor, honor, color
-ise becomes -ize: organize, realize, recognize
-ence becomes -ense: offense, defense
-gue becomes -g: catalog, prolog, epilog (although british forms are common too)
Thats it actually, how do you like this sentence, its my favorite: My neighbors favorite color is gray. Nich, huh? If u r from the u.k. the 4 differences will hurt your eyes.
By the way: thru, nite, lite and donut is just slang, its actually through, night, light and doughnut