Well, the other poster told you what the class is like - now as for "What is it about spinning?"...
As my thread states - I've only taken TWO spinning classes now (can only get there on mondays...) but honestly - I absolutely LOVED it! I was afraid to take it for the longest time because I've heard how "hard" it is...and yeah - it is - again, I almost passed out during last week's class...but yesterday I paced myself a bit more and felt SO exhilarated!!! yes, my butt hurt (lol) - but not nearly as bad as it did last week...i figure i can go get a gel seat cover to bring with me (they're fairly cheap)...and i don't think i'll invest in the shoes because the cages on the pedals work perfectly fine.
And you don't HAVE to do everything the class is during during your first couple classes - PACE YOURSELF and if you need to sit while everyone else is standing then do it - just make sure to keep the wheels moving.
But honestly - I absolutely LOVE spinning and I'm SO happy I've added it to my workout regime.