The resistance mechanism and settings of an elliptical trainer is very important in achieving your fitness goals. Treadmills use speed and incline to control the intensity of your workout. Elliptical trainers utilize resistance and the force required to move the foot pedals and the upper handle bars. This resistance is applied against certain muscle groups in both your upper and lower body. It causes you body to expend energy which results in burning calories and to some extend, building muscle mass.
You want an elliptical whose settings vary enough that it can handle a range of exercises from very easy to strenuous. This is particularly important if there are multiple users at different levels of fitness. You also want a machine that offers a range of resistance in small increments, so when you go from one level to the other there is not a very noticeable difference.
Elliptical trainer have many of the same types of programs that are utilized in treadmills, like hill training, cardio workout, fat burn, etc. For example, with a hill training program the level or resistance will vary from easy to hard, as if you were walking or running up and down hills. With heart rate control the resistance is determine by keeping your heart rate at a targeted zone.
All elliptical trainers use some form of magnetic resistance, where a spinning flywheel spins within a magnetic field. The stronger the magnetic field the more resistance to the foot pedals and handlebars. (Check out this guide to elliptical parts if you need clarification.)
Budget and mid-range machines utilize a manual or motorized brake system. There is a magnet caliper or fork with magnets attached. Resistance is generated by adjusting the distance of the magnets to the flywheel. The closer the magnets to the flywheel the greater the resistance to the flywheel and the cranks. The distance is either controlled by a motor or by hand.
More expensive units are built with eddy current brake systems. Resistance is controlled by the current fed to the electromagnet from the console. The stronger the current, the greater the resistance to the flywheel and cranks. The benefit of this type of brake system is there are no moving parts, which means there are no parts that can wear out. This resistance system is low maintenance with a low probability of any malfunction. It is also extremely accurate and very quiet. Motorized and manual systems are less durable and tend to be noisier. You will find the eddy current system on most commercial grade ellipticals.