The Windows XP official term for what was formerly known as the system tray: that is, a place to collect small icons near the clock in the lower-right corner (usually) of the screen. Tray icons most often serve a dual purpose: they notify the user about something's status (network connectivity, battery life, volume muting, etc) and respond to mouse clicks to perform various actions. The system notification area was first reachable without the use of a mouse in Windows 2000, by getting to the Desktop and hitting SHIFT-TAB. The arrow keys maneuver around the icons; hit SPACE for a left-click, ENTER for a double-click, and SHIFT-F10 (or that funny-looking menu key on your keyboard) for a right-click.
About a half hour after starting your first Windows XP session, a balloon reading "Hiding your unused notification icons..." appears as a very smooth animation hides the icons which you have not clicked at all. A small button on the edge of the notification area will show all of the hidden icons. This feature can be disabled by right-clicking the Taskbar and clicking Properties. The "Hide inactive icons" option can be disabled entirely, or settings (Always hide, Always show, or Hide when inactive) can be chosen for each icon that has appeared so far.
Overall, the system tray has been an inconsistently implemented idea (and I'm being pretty generous there) but at least Windows XP has dealt with the problem of tray trash fairly well.