An exercise bike can be a godsend if you, aren't fortunate enough to live in a temperate climate or, are one of the hearty souls that ride in any kind of weather.
To be honest, I don't care to ride in the dead of winter. But I don't like to start from scratch every spring either. So, my exercise bicycle is definitely my friend.
The design of an exercycle has come a long way since my first exercise bicycle about 30 years ago. It only had a speedometer, odometer,a very crude resistance device, and that was it.
Features that were only usually found in a gym are now on even basic models including variable resistance, heart rate sensing, time, speed, distance, calories, RPM, minutes per mile, LCD readouts, etc.
Many have built in programs so you can choose the exercise program that’s best for you at your current training level and change it to suit as you progress.
The most common types of systems used to vary the pedal resistance are....
Direct Tension - offers manual adjustment of resistance
Fly wheels - similar to a regular bicycle peddling motion
Air - resistance is achieved by peddling against the flow of air from a fan blade (in my opinion these are way to noisy)
Magnetic Frictionless Resistance - the most sophisticated type of resistance, allows for more variation in level of workout
I have found the magnetic resistance to be preferable as well as the quietest. If you intend to watch TV, or listen to music, you will want a quiet bike.
Upright bikes are the most popular and give the user an upright and comfortable riding position similar to a comfort or cruiser bicycle.(as opposed to a road bike)
You can stand and pedal, if you choose, when the pedal resistance is set at a high level or if your simulating a programed ride with varying pedal resistance.
A recumbent exercise bike was only found in a gym not too long ago and they were huge. Now,not only are they are commonplace, they are smaller and have some advantages over an upright bike. They are very easy on the back, butt, and hands.
This does not mean that they are easier to pedal but, since you can't stand in the pedals, your legs and butt get a better workout.
Women, especially, like the way they tone their legs and butt.
If you are a road bike kind of guy/gal an Indoor Cycle exercise bike is for you. It gives your home exercise cycling the feel of a real road bike.
The weighted flywheel builds speed like an outdoor bike to give you the feel of real cycling.
The stronger steel construction lets you stand up and pedal hard like you are riding uphill or just pedaling all out. This lets you work hard to burn calories and tone legs, hips, thighs, and buns.
If you have a bike now, and are thinking about an exercycle, you will need to decide if you want to simulate the riding that you have been doing or consider the options that would be useful for training purposes.
Upright bikes are the most popular and give the user an upright and comfortable riding position similar to a comfort or cruiser bicycle.(as opposed to a road bike) 
A recumbent exercise bike was only found in a gym not too long ago and they were huge. Now,not only are they are commonplace, they are smaller and have some advantages over an upright bike. They are very easy on the back, butt, and hands.
If you are a road bike kind of guy/gal an Indoor Cycle exercise bike is for you. It gives your home exercise cycling the feel of a real road bike. 
