For sending Notification to any android Device you can use two technology:
1) Push.
2) Pull.
For Push Technology you can use GCM(Google cloud messaging).
For Pull Technology you can make you application continuously keep on connecting to server and trying to fetch data if it is available from there.
Advantages of GCM:
1) Low battery comsumption. Since it will push the message to user device and it will push message when user gets connected to server.
For GCM you can use following Link for your reference:
1) http://developer.android.com/google/gcm/gs.html
2) http://www.techrepublic.com/blog/app-builder/implementing-googles-cloud-to-device-messaging/428
3) http://www.vogella.com/articles/AndroidCloudToDeviceMessaging/article.html
4) https://github.com/teleknEsis/TechRepublic-Samples/tree/master/C2DMSample
