Prepare job summaries, if relevant. You can use the job description template you use for each position that you advertise by filling in the template with the information that is relevant to that position. However, if you do (or expect to do) a lot of hiring of a particular position, you might make a special template that includes a summary of that job. This will save you some time.
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.