At the top of your screen, you will see notifications which report calendar events, alarms, incoming mails or other ongoing events. Swipe down from the top of the screen to reveal the list of notifications, then swipe up from the O-sign to hide it.
Depending on the notification type, you can swipe a notification sideway to dismiss it, or tap the X-sign to dismiss all from the list.
To access a notification, tap it and you're taken to the related email, reminder, message or app. The notification then clears automatically from the list where applicable.
Note: When you slide down from the top of the screen, you can also see this Quick Settings icon. Touching this icon is equivalent to running the Settings app.