Usually there's one or two job responsibilities among many, that you can do, but not the way everyone else does them. The ADA requires employers to make reasonable accommodations, and some employers don't want to.

So less, the job requires you to stand all day, and more, once a week or so they ask you make a binder of materials, and the hole puncher they want you to use dislocates your hands (true story). Or, it's a desk job, but you can't get from your desk to the bathroom in your wheelchair unless they widen the aisles between desks (hypothetical).