Functional reactive programming is an idea from the late 90s that inspired Erik Meijer, a computer scientist at Microsoft, to design and develop the Microsoft Rx library.

Rx is a reactive extension for Microsoft .NET. Rx provides an easy way to create asynchronous, event-driven programs using Observable sequences. A developer can model an asynchronous data stream using Observables, query the Observables using LINQ syntax, and easily manage concurrency with Schedulers.

Rx makes well-known concepts, such as the push approach, easy to implement and consume. In a reactive world, we can't just wait for a function result, a network call, or a database query to return and pretend that the user won't notice or even complain about it. Every moment we wait for something, we lose the opportunity to do other things in parallel, provide a better user experience, and free our software from the chains of sequential, blocking programming.

.NET Observable relates to .NET Enumerable according to the following table:

The push approach reverses the problem: instead of asking for a result and waiting, the developer simply asks for a result and gets notified when the result is available. The developer provides a clear sequence of reactions to the events that are going to happen. For every event, the developer provides a reaction; for example, the user is asked to log in and submit a form with his username and password. The application executes the network call for the login and states what is going to happen:

As you can see with the push approach, the developer doesn't wait for the result. He will be notified when the result arrives. In the meantime, he can do whatever he wants: