There are lots of big changes in Windows 8 and one of the less obvious ones is notifications. In the past you may have used third party software to notify you when a new email arrived. It wasn't perfect and a lot of people didn't even have email notifiers. If you used a calendar to store appointments and meetings, how did you know when they were about to take place? This also requires a notifier of some sort.
Windows was lacking in this respect, but now the situation has changed. Many apps are able to display notification messages on the screen and while some can be useful, others are not. You might be busy working or playing and not want to be disturbed with notifications. You can choose which applications are allowed to display notifications on the screen and you can choose to have a break and block notifications for a short time.
If you want to block notifications temporarily because you don't want to be disturbed, move the mouse pointer to the top right corner of the screen to show the charms, drag down and click Settings. Click Notifications and choose Hide for 1, 3 or 8 hours.
To choose which applications are able to display notifications, press Windows+C for the Charms, click Settings, Change PC Settings. Select Notifications on the left and on the right are all the notifications options.
App notifications can be on or off, which is like a global switch. Below are on/off switches for individual apps. Apps like Mail, Calendar and Messaging are standard built in Windows 8 apps, but if you have downloaded software from the Store, there will be others. It's your choice whether to let them show notifications. Sometimes, they have the ability, but don't make use of it.