According to Aurora University, a job description "describes the purpose, duties, responsibilities, tasks and relationships of a particular job." While the obvious purpose is to let your potential applicants know a position is available and what the nature of the job is, job descriptions also serve legal and organizational purposes and must be carefully written to avoid lawsuits or internal conflicts.
The external job description is the one you post for potential applicants. It lists the title and essential functions of the job, outlines duties and responsibilities and may include administrative information such as the responsibilities of the overall department and the position of the job's supervisor. It should also list necessary qualifications, including skills, education and experience. Most external job descriptions indicate the salary and benefits offered for the position. Though they need to be brief, they should also be specific so that you're not inundated with applications from people who are unqualified.
A generic or general job description describes the job in broad terms. Depending on the size of the organization and the number of similar job positions within it, the generic description may be used as a template for department heads to craft more specific descriptions for jobs under their purview. However, the Poindexter Consulting Group warns that generic job descriptions can open a company to problems with the Equal Employment Opportunity Commission, which enforces the Americans with Disabilities Act. This act mandates that employers not discriminate against qualified disabled people who are able to fulfill the "essential functions" of jobs. If a generic description doesn't detail what the essential functions are, you could create the appearance of discrimination.
Moreover, the government uses job descriptions to determine that employers are following legal guidelines regarding equal pay and opportunities for overtime. Generic descriptions that don't specify wages and hours won't protect you if your organization comes under government scrutiny.
The internal job description contains the same information as the external one but goes into more precise detail, according to the Grand Roads Executive Search firm. The administrative information, for example, may include the name and job title of the position's supervisor. Internal descriptions of higher level jobs may list metrics such as how much revenue the jobholder is expected to generate, how many clients or accounts she will oversee or service or how many employees she will supervise. A well-written, thorough job description ensures everyone knows what your expectations of the position are so that Human Resources can hire the right person, the person hired understands what to do and you're legally protected if the new hire doesn't meet those expectations.