This is an offshoot of the ridiculous “A Day At The Gym” thread in the BBQ Pit. Hopefully we won’t debate the sexuality of this device or it’s users.
It’s a machine found in contemporary gyms. You stand on two elongated treads/pedals and walk. “Elliptical” has to do with the back end of each tread, which is affixed to an elliptical rotor so that when you step you get an up and down motion. It’s supposed to be easier on the knees than a stair stepper. IIRC, Consumer Reports said they burm the same number of calories as a treadmill. In my experience, they’re more exhausing to use. Put “elliptical trainer” into Google or go to the sears exercise equipment page for a photo.
just an example. There are many different styles
Ah, the elliptical trainer. Affectionately nicknamed The Orbitron of Death. It is a great device for cardiovascular exercise, and much loved by people with injuries or infirmities. It is also much reviled by people who can’t master it. 
I busted up my left knee something fierce back in the service, and while jogging/running outdoors or on a track is no problem, treadmills and stair steppers make my knee feel very “floaty” after extended periods. The elliptical trainer allows one to “run,” either forward or backward, with much less stress and (I think) with greater range of motion. And it’s tough.
If you should discover one at your local gym, it takes some getting used to, so give it at least three tries. After you’ve “run” forward for about 15 minutes, try switching to reverse. You’ll discover muscles you never even knew existed.
My favorite is the Bataan 4000 Death Marcher.
These things are hell on the quads. I was in serious burning pain after two minutes.
Some facts on elliptical trainers, from a firm believer:
-Contrary to treadmills, they cause no impact to joints whatsoever, as your foot is always in contact with the pedal.
-Those that offer “ski poles” to push/pull will use the greatest number of muscles simultaneously of any cardio machines, and thus, allow a greater calory burn rate for a given heart rate than any other cardio machine. More than rowing, more than plain x-country ski machines, because your feet go up & down as well.
-Different models have differently shaped “ellipses” describing the trajectory around which your feet travel. Those with a higher/fatter ellipse will feel more like a stairmaster. Those with a flatter/longer ellipse will feel more like running in slow motion in mid-air. I have christened that feeling the “6-Million Dollar Man” feeling. cue theme music… You need to find one that feels right for you…
-because they involve so much muscle mass & are so efficient at burning calories for lower levels of effort, are low impact, don’t require a minimum speed, and can be used at very easy levels at first, they are often the cardio method of training of choice for people who are just starting to exercise after long periods of being sedentary, recovering from chronic illness, or heart disease. In such a case, I would also recommend that you follow a heart-rate controlled work out intensity. In other words, using a heart-rate monitor, you exercise to keep you heart rate in a training “range” determined by your doctor / trainer. If your heart rate goes above, you slow down to bring it back in the range. This was simultaneously recommended to me by both our family MD, and the trainers at my work gym.
The elliptical and heart-rate controlled training allowed this chronic asthmatic to attain levels of cardio fitness that I would have said, 4 years ago, were about as likely as my going to the olympics.
Hope this helps.
Good to know, trupa. I hope a physical therapist or an MD will weigh in as well.
Mmmm-hmmm. A big hit in the Philippines. 
When I have to use a stair stepper, I prefer Bruce Willis’s Die Hard model. While you exercise, a well-dressed German man points a gun at your head and taunts you with mispronounced puns. The one at my gym really looks like Alan Rickman.