This is a .Net feature that allows programmers to extend classes or interfaces with new methods even when they do not have the source code for the class or interface they want to extend.

The OWIN Framework defines extension methods to:

• Provide a fluid syntax for declaring and configuring middleware to add to the OWIN pipeline.
• Store and retrieve objects in the OWIN context based on the interface that they implement.
• Shorten Guids.