A stationary bike is used for indoor cycling, and it has similar characteristics to what you would see on a road bike. A typical class includes an instructor motivating the participants to keep going while providing instruction, while high energy music is played in the background. The pace will usually vary quite a bit from very fast to slow. Every so often, you will even pedal standing up. Those who participate in cycling competitions will find that the variation of pace and intensity does prepare the body and mind for such events.
Indoor cycling is one of the best exercises for burning a high number of calories in a short period of time. In a typical 30 minute session, the average person can expect to burn around 500 calories. Anytime you exercise at an above average intensity level, you will burn a lot of calories. Not to mention that indoor cycling is fun since you get to be around other people.
If weight loss is your goal, the indoor cycling is one of the better exercises you can do. Perhaps you've heard an instructor or trainer talk about being in the "fat burning zone". This means that your heart rate is at 60-70% of its maximum. While you will certainly burn more calories in this zone, you will burn much more when it's in the 70-85% range. The intensity varies throughout an indoor cycling class, so your heart rate will as well. Sometimes you'll be in the fat burning zone, other times you'll be working at a higher intensity.
As with any exercise, indoor cycling has its positives and negatives. It's ideal for cyclists who need to train for a race, but need to be indoors because of inclement weather. In general, it's a great exercise for getting fit. You can choose how intense you want the workout to be. One day could be challenging, and the next could be more for recovery. Another benefit is that it doesn't place hardly any stress on the joints, as opposed to jogging.
The negative however is that being inside just isn't the same as getting to ride outdoors on the open road. Because you are stationary, it's much more difficult to challenge yourself to train as hard as possible.
If you are just starting out, then it might be difficult to get through the first few sessions. After awhile, your body will begin to adjust and that's when you'll notice the results.