The job description is an accurate description of the role, responsibilities, and necessary qualifications of a job. It is usually a dry piece of technical copy, meant for internal use to pass along information about job requirements and to give expectations to a new hire.
A job posting is meant to work as an advertisement. It treats potential recruits like great customers, gets them excited about the possibility of working for your company, and helps you quickly fill a job vacancy.
A job requisition is a preliminary step to making a posting but often comes after the description has been created.