Use Streaming API to receive notifications for changes to Salesforce data that match a SOQL query you define, in a secure and scalable way.

Streaming API is useful when you want notifications to be pushed from the server to the client based on criteria that you define.

For simple example, check the below link

https://www.infallibletechie.com/2016/02/streaming-api-example-in-salesforce.html

For more information, check the below link

http://www.salesforce.com/developer/docs/api_streaming/

Cheers!!!