The retrospective meeting occurs on the last day of the sprint, after the sprint review meeting. In this meeting, your team inspects and explores how it has been working within the Scrum processes. Based on this analysis, your team might decide to adapt its processes to improve its own effectiveness, productivity, quality, and satisfaction. This meeting and the resulting improvements are critical to the agile principle of self-organization. For more information, see Agile Principles and Values, by Jeff Sutherland.
If your team did not complete all of the user stories that were assigned to the sprint, you will discuss why that happened in the retrospective meeting. Your team will determine whether it can adapt its processes so that those kinds of problems are less likely to occur. Also discuss issues that affected your team's general effectiveness, productivity, and quality and your team's satisfaction with the project.
For example, consider a team that had several tasks that only one individual on the team could perform. That isolation of expertise created a critical path that threatened the sprint's success. The individual team member had to work very long hours during the sprint, and the other team members were frustrated that they could not do more to help with those tasks. That team decided to practice eXtreme Programming to help correct this problem over time.
In some cases, your team may need to do some work to implement an improvement. For example, consider a team that was spending too much time dealing with failed builds. That team decided to implement continuous integration. The team did not want to risk disrupting their regular build process. Therefore, they allocated a few hours to set up a trial build before turning it on in their production build. The team created a spike and prioritized that work against the rest of the product backlog.
For more information about how to run effective retrospective meetings, see the following Web resources: Project Retrospectives: A Handbook for Team Reviews and Agile Retrospectives.