Spinning is a stationary bike class - you get on a bike and adjust the gears so it's like yr going uphill or downhill. You do different sections to fast music - racing, hill riding, up out of the saddle for steep hills etc. In my gym they do it to strobe lights etc - great fun and a fab calories burner. CV exercise is cardio-vascular - i.e. it makes you breathless, burns calories etc.
I don't think anybody needs to do any of the above if they are of average weight and they walk plenty. It's mainly for people whose day to day work prevents them from getting exercise I guess, or people who are trying to lose weight. I have to say, it did wonders for me when I was going thru a very bad patch emotionally - the gym kept me sane, filled up a couple of hours in the evening, got me out to meet people and kept me away from needing anti depressants (or alcohol!!). I wouldn't be without it.