Spin class is cycling with an instructor. My DH asked me, "why do you need a teacher to tell you how to ride a stationary bike?" I told him to come to class and see!
The instructor guides you through different "rides". You turn the tension up or down on the bike as she instructs, and you do endurance, mountain biking, hills, interval, etc. She takes you through the workout so that you get the best results basically. It was pretty fun but hard!
I am teaching childbirth classes 2x per week. The classes are 3 hours each. I think the hardest for me is being gone from the kiddos 2 nights per week. I miss dinner/bath/bedtime on those nights. DH is great with them, but I am gone for 4+ hours each night. Like I said, it could be worse. I could have to work full time like some moms do. But I am adjusting. I think after I finish the courses I am on now, I am going to take it back down to once per week though.