The ADA prohibits job discrimination against any qualified person who has a disability.
Employees or applicants have a qualified disability if they have "a physical or mental impairment that substantially limits one or more major life activities." The ADA also covers people who "have a record of such an impairment." Plus, people earn ADA protection if you regard them as having a disability, even if they don't.
To earn ADA job protection, people must be able to perform the job's essential functions, with or without a reasonable accommodation.
Employers and employees must work together in a good-faith, interactive process to choose reasonable ADA accommodations. Employers ultimately determine which accommodations are reasonable, but this determination can be second-guessed by the courts.
The ADA covers employers with 15 or more employees. Some states have laws similar to the ADA that cover smaller employers.