Spinning which I do 3x /week now as well is an intensive cardio workout of 45 mins each (at least here it is) with about 35-40 mins of effective cycling.
A group of people sit on stationary bicycles with a heavy flywheel that provides no resistance at all till a huge amt of resistance depending on the type of exercise..
An instructor sits in front of the class and tells people what to do and shows them how...
E.g. you sit down and turn pedals as fast as possible with no resistance. Or you stand up and pedal with very high resistance, or which is a killer for the hamstrings you alternatively sit down and stand up, alternating this every 1 second or so...
Mostly resistance is very high when standing up.. but at times resistance is low and you are told to try to spin as fast as possible while standing up..

Most beginners are completely exhausted by the end of the lesson, but if you're an experienced cyclist, you'll be recovered soon.. I just went from 2x/week to 3x this week (Mon-Wed-Sat).. am slightly addicted ;-). Will join the spinning marathon again , just like last year (4 lessons in a row: 3 hrs total).

Part of the lesson is very loud blearing music... it used to be awful, but now it's a bit better..
Other part is pre- & post-ride stretching which takes about 5-8 mins of the total lesson of 45 mins..
Some people don't break a sweat, others create small swimming pools below their bikes.. you're supposed to cover the bars with a towel . And I drink, drink, and drink.. mostly about 1,5 L in each lesson..
Caveat: when I don't add extra mineral salts, like eg Mg/Ca I get cramps later... which I tend to forget taking along a couple of times.. hence my plans to carb up on (nearly always quite salty) licorice.

It all started in the USA, but is now also (finally) quite popular in the Netherlands where I live...