Notification icons report the arrival of new messages, calendar events, and alarms, as well as ongoing events, such as when call forwarding is on or the current call status. When you receive a notification, its icon appears in the Status bar, along with a summary that appears only briefly. For a table of notification icons.

The Trackball light pulses once or repeatedly when you receive a new notification. Depending on your settings and the model of your phone, you may also hear a notification sound, the phone may vibrate, and LEDs may blink.  These settings and adjusting sound volume and other general notifications settings.

You can open the Notifications panel to view a list of all your notifications. Applications whose activities produce notifications, such as Gmail and Google Talk, have their own settings, which you can use to configure whether and how they send notifications, whether they sound a ringtone, vibrate, and so on.

To open the Notifications panel, Drag the Status bar down from the top of the screen. The Notifications panel displays your wireless provider and a list of your current notifications. Notifications about ongoing activities are listed first, followed by Notifications about events, such as new mail notifications or Calendar reminders.

Touch a notification to respond to it. The Notifications panel closes. What happens next depends on the notification. For example, new voicemail notifications dial your voicemail box, and network disconnect notifications open the list of configured networks so you can reconnect.

To clear all notifications, Touch Clear at the top right of the panel. All event-based notifications are cleared; ongoing notifications remain in the list. To close the Notifications panel, Drag the tab at the bottom of the Notifications panel to the top of the screen.
Or just press the Back button. The panel also closes when you touch a notification to respond to it.

Google Android 2.2 User’s Guide