WCF (Windows Communication Foundation) is the name that Microsoft uses for a Framework that can Communicate between multiple servers independently of their Operating System or Technology.
You can use this framework to send and receive messages and attachments (files, images, etc) to and from any computer in the world.
From a developer point of view, it's the new version of WSE (Web Services Enhancements) with MTOM (SOAP Message Transmission Optimization Mechanism) that was itself an updated version of the simple Web Service (.ASMX).
WCF is called a Service Framework and not a Web Service because it no longer depends on the IIS (Internet Information Server) to be hosted. Now you can host a WCF application using TCP/IP, host it as a Windows Service or even host it as an application - and you can still host it in IIS as well.
It uses the SOAP protocol and others (this is configurable in just one file), and supports HTTP and HTTPS as well, for messaging and attachments. You now have plenty of new security options with it... and it's the "top-of-the-line solution" to use to send/receive messages from anywhere to anything.
It is heavily used in the AJAX world, because it can send messages in the JSON format. Because of this, jQuery and other JavaScript frameworks work lovely with it.
For Reading material, I recommend buying a good book, because it's a good way to do self-learning and have it for reference at a later time. I personally bought Wrox Professional WCF, and I'm loving it.
