A job advertisement is sales copy, except the customer is the candidate. There are three most common mistakes. The first is a list of requirements longer than the description of the work, which puts off particularly good candidates, because they will not apply if they do not meet everything, while less suitable candidates apply regardless. The second is not stating the salary, or stating the statutory minimum, which, when offers are being compared, comes across as a signal that something is being hidden. The third is describing the company at the start instead of describing the job. An effective advertisement starts with what the person will be doing and with whom, continues with what they actually need to know, states the salary range, and ends with a clear next step, including by when you will get back to them.
See also: Careers page, Job description, Employer Branding.