The task bar was introduced with Windows 95 and has been part of every version of Windows since then. It is the bar that spans the bottom of the screen and contains the Start button on the left side and the systray on the right. The task bar also includes the current time on the far right side and can hold shortcuts to programs directly to the right of the Start button.
Most of the task bar, however, contains shortcuts to open windows. Whenever you open a program or window, it shows up in the task bar. So if you have Internet Explorer and Microsoft Word open, there will be at least two items in the middle area of the task bar. Since each window gets its own location in the task bar, if there are two Microsoft Word documents open and three Internet Explorer windows open, there will be a total of five items in the task bar. When you click on a window title in the task bar, that window will become active and show up in front of other open windows. You can also cycle through the task bar items by holding the Alt key and pressing Tab a few times.