What is Durandal and how does it relate to Windows Phone development?

If you are browsing the Windows Phone development code samples on MSDN, you might run into something called Durandal.

What is Durandal and how does it relate to Microsoft's phone development? Durandal is an open-source project hosted on Github, Durandal allows building single-page applications (SPAs) using JavaScript, and thus, can also be used on Windows Phone applications.

For more information, check out the home page of the project.