A home inspector is the "general practitioner" of the home inspection field. Some home inspectors are also specialists in specific areas such as energy efficiency, structural engineering, architecture, or environmental assessment. Specialty inspectors advertise their area of interest and usually restrict their work to that specialty.
Several states require home inspectors to be licensed. In the State of Washington, home inspectors must be licensed Pest Inspectors in order to be able to inspect for wood destroying organisms and conditions conducive for the growth of such organisms.
There are numerous schools and inspection organizations throughout the country, and thousands of inspectors. Because the quality and purpose of the organizations vary, so do the qualifications and skills of inspectors. Many home inspection associations have been spending too much time and money marketing to the trade and the real-estate industry and not enough effort in training their members.
The inspector you choose must have the technical expertise needed to analyze the home, plus the communication skills to convey the results.