Spinning is a phenomenon, where participants take part in a group workout on exercise bikes that typically lasts anywhere from 30-75 minutes. The classes are lead by instructors who normally guide participants through a series of phases, from a gradual warm-up to a period of peak effort which is then followed by a gradual cool down. In spinning classes, the intensity of the workout is influenced by a couple of things:
cadence, or pedal rate
resistance of the bike's flywheel, which can be continually adjusted throughout the class to make pedaling easier or more difficult
the riders' body position, as they either pedal from a seated position or rise from the saddle
Ultimately particiapants determine their own levels of exertion, something that works better for some folks than others.
But a good instructor can certainly encourage and motivate you to push yourself, and you'll most likely find spinning to be a vigorous workout. It's a decent way to stay in shape when you can't get outside on the bike.