The Americans with Disabilities Act (ADA) prohibits discrimination against employees (and job applicants) who have physical or mental impairments that substantially limit "major life activities." Major life activies include walking, sitting, reading, seeing, and communicating. With the ADA Amendments Act in 2008, Congress made it clear that the ADA should be interpreted broadly to include many disabilities, to offer protection to as many people as possible.
An employer doesn't have to hire or retain someone simply because that person has a disability: The person must be qualified for the job. This means that the person must have the necessary degrees, licenses, experience, and so on for the job. It also means that the person must be able to perform the essential functions of the job, with a reasonable accommodation, if necessary.