Banners, Alerts, and Badges
Notifications work with your apps to let you know when something has changed. For example, you can see when there's incoming email in Mail, or upcoming appointments in Calendar. Each app displays these notifications as a banner, alert, or badge.
Banners appear in the upper-right corner of your screen, without interrupting what you're doing. Banners appear for a short period of time, then slide off screen to the right.
On the left of the banner is the icon of the app that generated the notification. The rest of the banner contains a brief message from the app. You can click the banner to open the app that generated the notification.
Alerts are important announcements that appear similar to banners. Alerts remain on your screen until you take action. Many alerts include buttons you can use to interact with them without having to open the app that generated them.
For example, you can reply to a chat directly from a notification.
Or if you receive a FaceTime call you don’t have time to answer, simply reply with a message or set a callback reminder.
To dismiss an alert notification without interacting with it, use your pointer to swipe the banner to the right.
Badges keep a count of how many new items or notifications are waiting for you in an app. A badge with a number appears on the upper-right of an app's icon in the dock to indicate how many new items are waiting. For example, a badge attached to the App Store icon indicates how many updates for the software you’ve installed from the App Store are available to install. A badge on the Mail icon in the Dock tells you how many unread emails you have.
Notifications can even appear on the lock screen, or while your screen saver is active so you stay informed.
Notification Center
With Notification Center, you can see all of your notifications at a glance. To open Notification Center, click its icon 
in the menu bar, or swipe left with two fingers from the right edge of your trackpad.
All of your recent notifications are listed in the Notifications tab. You can quickly respond to outstanding messages or emails directly from the Notifications tab. You can also click a notification to open its related app.
The Today tab in Notification Center shows your day at a glance.
Each section of the Today view is a widget that provides a focused view of a particular topic. OS X Yosemite includes the following widgets:
- Today
- Tomorrow
- Calendar
- Calculator
- Reminders
- Social
- Stocks
- Weather
- World Clock
Add, remove, or rearrange these widgets by clicking the Edit button at the bottom of Today view.
Click the Add button (+) to add a widget. Click the Remove button (-) to remove a widget. When you're finished, click Done.
You can install additional widgets by installing apps from the Mac App Store that include them.
Do Not Disturb
If you don’t want banners to appear during certain times, you can temporarily turn off notifications using Do Not Disturb.
You can toggle Do Not Disturb by clicking the Notifications icon while pressing the Option key. You can also open Notification Center and scroll down with your mouse or trackpad to reveal the Do Not Disturb switch. Click the switch to toggle Do Not Disturb on or off.
To set a specific schedule for Do Not Disturb, use the Notifications pane in System Preferences.
Learn More
You can change how apps notify you from the Notifications pane in System Preferences. Choose System Preferences from the Apple menu, then click the Notifications icon. For more information about Notifications, click the Help button 
in the Notifications preferences window.
FaceTime is not available in all countries or regions.