A well written position description, depending on the job, may include the following:

Job:
A brief statement or summary describing the objectives and purpose of the job.
A clear description of the tasks, duties and responsibilities involved.
Reporting relationships.
Authority of the position - extent of incumbent's delegated authority eg access to files held by the departmental manager, or approving petty cash expenditure.
Accountability - the criteria against which performance is judged.
Conditions of employment - hours, leave, salary, employment status.
Location of position.

Person:
Qualifications.
Experience.
Technical skills, computer skills, languages spoken.
Personal qualities - organisational skills, time management, communication skills.
Problem solving and decision making.
Special requirements - eg drivers licence.

These lists are not exhaustive but used for illustration only.