A job posting should sell your employer brand and the role to attract candidates of the highest calibre. You do not need to outline every duty but you should add sufficient detail to entice applications, including approximate pay information. Being too vague will make applicants think you have something to hide. Using search engine keywords in the text will make your advert easier for the appropriate candidates to find.
Specify if the position or department is new to encourage applicants interested in building success. Give details of your expectations and any potential challenges to attract high achieving candidates and weed out those who will find the role too hard. If you require candidates to think outside the box or to demonstrate particular qualities, highlight this in your advert and use both a style and language that makes this evident. Talk up selling points such as a great benefits package, training opportunities or a fun work culture.
Clearly defining the role and your expectations of candidates will attract suitable applicants who will add value to your firm whilst simultaneously discouraging time-wasters.