WCF is an SDK for developing and deploying services on Windows.
WCF provides a runtime environment for your services, enabling you to expose CLR types as services, and to consume other services as CLR types. Although in theory you could build services without WCF, in practice building services is significantly easier with WCF. WCF is Microsoft’s implementation of a set of industry standards defining service interactions, type conversion, marshaling, and various protocols’ management. Because of that, WCF provides interoperability between services. WCF provides developers with the essential off-the-shelf plumbing required by almost any application, and as such, it greatly increases productivity. The first release of WCF provides many useful facilities for developing services, such as hosting, service instance management, asynchronous calls, reliability, transaction management, disconnected queued calls, and security. WCF also has an elegant extensibility model that you can use to enrich the basic offering. In fact, WCF itself is written using this extensibility model. The rest of the chapters in this book are dedicated to those aspects and features. Most all of the WCF functionality is included in a single assembly called System.ServiceModel.dll in the System.ServiceModel namespace.
WCF is part of .NET 3.0 and requires .NET 2.0, so it can only run on operation systems that support it. Presently this list consists of Windows Vista (client and server), Windows XP SP2, and Windows Server 2003 SP1 or their later versions
http://msdn.microsoft.com/en-us/library/aa702682.aspx
http://en.csharp-online.net/WCF_Essentials%E2%80%94Services
Comparing WCF with WEbservice: Just check this link which will tell you all infor abt same;
http://msdn.microsoft.com/en-us/library/aa738737.aspx