Explain various dashboard components in Salesforce?

Various Dashboard Components are:

1. Chart: Use a chart when you want to show data graphically.

2. Gauge: Use a gauge when you have a single value that you want to show within a range of custom values.

3. Metric: Use a metric when you have one key value to display.

Enter metric labels directly on components by clicking the empty text field next to the grand total.
Metric components placed directly above and below each other in a dashboard column are displayed together as a single component.
4. Table: Use a table to show a set of report data in column form.

5. Visualforce Page: Use a Visualforce page when you want to create a custom component or show information not available in another component type.

6. Custom S-Control: Custom S-Controls can contain any type of content that you can display or run in a browser, for example, a Java applet, an ActiveX control, an Excel file, or a custom HTML Web form