Think of this as describing the purpose of the position, and its role within your company or organization as a whole. A job summary will briefly acknowledge the fundamentals of the position, so leave space on your job description template to answer.
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.
The purpose of a job description is to persuade, as well as to qualify and disqualify candidates before you spend time interviewing them. If you’re looking for someone with a degree in economics, then putting that on the job requirements list will disqualify the candidates without the necessary qualification. On the other hand, the right copy and tone can help you stand out to incredibly desirable candidates.
The term ‘job description’ is a flexible one, but here I’ll be using it in a modern context, to describe all of the copy you’d use on an online job board. It usually includes information like job title, duties, salary, etc.