What is asynchronous Programming?

In asynchronous programming, the main program continues to execute while other timebound tasks finish their execution, it doesn't wait the other child task to complete.

For example, Let's say, there is a user interface and user click a button to get some data through an external webservice call. In typical asynchronous execution, the application triggers a separate thread in the background to call the webservice to fetch data and meantime the main user interface thread remains responsive for user interaction.