OS X Yosemite: See your notifications
Notifications let you know what’s up. For example, when you get a new email or a FaceTime call, you get a notification in the top-right corner of your screen. You even get notifications from websites when Safari isn’t open. And when you’re away (when your display is asleep or locked), you can see all the notifications you missed on the login screen when you get back.
See all of your recent notifications: Click the Notification Center icon 
in the menu bar, or swipe left with two fingers from the right edge of your trackpad, then click Notifications.
See more about a notification: Click Show in a notification, or click the notification. You can also open Notification Center, then click a notification.
Respond to email or text messages: Click Reply in the Mail or Messages notification.
Respond to calls: Click Accept or Decline in the notification. You can also click the disclosure triangle next to Decline, then choose Reply with Message to instead reply using the Messages app.
Choose when to update software: Click Later in the notification, then choose a time. For more information about updating, see Keep your Mac up to date.
Repeat a notification: Click Snooze in the notification.
The notification reappears fifteen minutes later. If you want to choose when the notification reappears, click and hold Snooze, then choose a time.
Temporarily turn off notifications: In Notification Center, click Today view, scroll down, then click to turn on Do Not Disturb. You can also Option-click the Notification Center icon in the menu bar.
You can also turn off notifications for a specific app or turn off all notifications during specified times.