A Summons is a document or writ directing the sheriff or other officer to notify a person that an action has been commenced against that individual and that he or she is required to appear, on a certain day, and answer the complaint in such action.
What is a writ?
An official court document, signed by a judge or bearing an official court seal, which commands the person to whom it is addressed, to do something specific. That "person" is typically either a sheriff (who may be instructed to seize property, for example) or a defendant (for whom the writ is the first notice of formal legal action. In these cases, the writ would command the person to answer the charges laid out in the suit, or else judgment may be made against them in their absence).