Can someone explain to me schools that go by semesters?

Next year I’m attending a school that goes by semesters. It goes by 12 weeks, some classes go by 6 weeks, some go by 3 weeks. Most of the classes I need to take are only 6 weeks long.

Why are some 12 weeks, some 3 weeks, and some are 6 weeks?
I’m used to taking many credit hours, I took 17 credit hours at my school that goes by quarters. Would it be difficult to take 3 classes at once for 6 weeks??