http://www.hhs.gov/ocr/privacy/hipaa/understanding/coveredentities/

The HIPAA Rules apply to covered entities and business associates. A Covered Entity is one of the following:

1. A Health Care Provider

This includes providers such as:

Doctors
Clinics
Psychologists
Dentists
Chiropractors
Nursing Homes
Pharmacies

…but only if they transmit any information in an electronic form in connection with a transaction for which HHS has adopted a standard.

2. A Health Plan

This includes:

Health insurance companies
HMOs
Company health plans
Government programs that pay for health care, such as Medicare, Medicaid, and the military and veterans health care programs

3. A Health Care Clearinghouse

This includes entities that process nonstandard health information they receive from another entity into a standard (i.e., standard electronic format or data content), or vice versa.