Definition
A job description describes a role in a stable way: its purpose, main responsibilities, required skills and experience, working conditions and direct supervisor. It is used for hiring, but also for performance reviews, pay equity and training.
It is not the same as a job posting, which is a public text designed to attract candidates. The job description is its factual basis.
Best practices
Update it before each hire, during the intake meeting. Clearly separate must-have requirements from assets: every unnecessary requirement (an unneeded degree, arbitrary years of experience) shrinks the candidate pool without improving the hire. In Canada, the National Occupational Classification (NOC) can help name and position a role.
Concrete example
The description for a lab technician role required a bachelor's degree. During intake, the manager admits a college diploma and two years of experience are enough for the actual work. Updating the description widens the candidate pool without lowering any requirement that matters. The job posting then inherits the corrected requirements.