The Sprint is the core of Scrum. It's a finite block of time used for focused work with the goal to produce a potentially shippable product increment at the end of it. A Sprint starts with the Planning and ends with the Sprint Review and Retrospective. A very commonly choosen length for a Sprint are two weeks but it could be even less or more, though not more than four weeks. Sprints start immediately after each other and always with the same parameters.