After opening Reporting Builder, you need to connect to Report Server so you can use existing Data Sources or Share Data Sets in your reports. You can create embedded Data Sources as well if you like.
By using Report Builder, user can create different type of reports such as Tablix, Chart, Matrix and Map etc.
If user has publishing permission, he/she can save the report to report server from Report Builder.
User can also open existing reports and make changes and save to report server as well.