Normally double-clicking these will start the associated application.

 

Click this button to open the Start Menu, and select an item to start an application.

 

Normally clicking these (with a single click) will start the associated application.

 

This shows the "status" of your computer.  Windows will highlight application icons in the task bar to indicate they are running.

 

This is where a running application will display its window when it is in its "normal" or "maximized" state.  See Window State for more information.

 

Normally on the right side of the Task Bar, the System Tray shows the status of some services that are running.  Left- or right-clicking the displayed icons can display further information, pop up a menu, or pop up an application GUI to interact with those services.