A job description is an internal document that clearly states the essential job requirements, job duties, responsibilities, and skills required to perform a specific role. A more detailed job description will cover how success is measured in the role so it can be used during performance evaluations. They are also known as a job specification, JD, and position description (job PD).
A job specification could be considered a more precise job description that details the exact educational degrees, experience, skills, and requirements for a role. In most cases, these terms are used interchangeably and nearly always describe the same document.