Scrum timeline

The first sprint can start when the business representatives have agreed to build something for the organization, providing a Vision Statement (idea, goal, vision) and a Product Roadmap (top level deliverables) that defines and describes the idea and the goal of the project.
The Vision Statement and the Product Roadmap are not part of Scrum, but are essential parts of managing projects and are covered in other Agile frameworks.

Pre-Sprint Activities

What happens prior to the Sprints (Pre-Sprint):

The Vision Statement provides a concise description of the goals of the project which help the team stay focused on what is important from the organization point of view.
The Product Roadmap is an initial visual timeline of major product features to be delivered and is normally created by the Product Owner.
Stories are user requirements which will be turned into deliverable features, normally written by the Product Owner based on requirements from the customer.
All these stories make up the Product Backlog and the first Sprint can start as soon as there is enough stories defined, without the need for the Backlog to have 100% of the details.

Sprint Activities

What happens during the Sprints:

Sprint Planning meetings are held to plan what will go into a Sprint (a fixed period of time used to deliver parts of the final product). The Product Owner prioritizes these requirements and therefore decides on the contents of the Sprint Backlog.
These stories (features, functionalities or deliverables) make up the Sprint Backlog, so the Sprint Backlog is a list of all stories that will be developed in the next Sprint.
The Team breaks down (expands) these stories into tasks.
The Team then takes 2 to 4 weeks to deliver an agreed amount of stories.
The Team holds a Daily Scrum meeting (standup) of 15 minutes each day to collaborate with each other.

Post-Sprint Activities

What happens towards the end of the Sprint:

At the end of the Sprint, the Team demonstrates the completed stories (products) to the customer in a Sprint Demo (aka Sprint Review) meeting.
The last activity is the Scrum Retrospective meeting, where the team reviews the Sprint and looks for ways of improving it, integrating lessons learned (e.g. 3 items to change the following sprint). The Scrum Master ensures that the Scrum process is followed entirely and offers coaching to everyone involved.