What is the role of Report and Dashboard in Salesforce ?

A report is a template which is used by users to display the information in well defined form. Report is displayed in tabular or graphical chart. It can be filtered, and grouped. Also you can organize the reports in a Folder. User can control the access on this folder by keeping public, hidden, shared or sometimes read only access. One can made the folder is accessible for entire organization or vice versa can be available for single person based on permissions.
Salesforce Dashboards
Dashboard is a page layout for visual display of your data. Each dashboard has 1:1 mapping for its component and report. One can clubbed multiple dashboard components to display a single report. The security settings is used to control the visibility of data. These dashboards are dynamic in nature. User can only view their own data as per their settings.