C# .Net WCF (Windows Communication Foundation) and Web API are all technologies developed by Microsoft that are used for building applications and web services.
C# .Net is a modern objectoriented programming language that is designed for building applications that run on the .NET Framework. It is one of the most popular languages for developing enterpriselevel applications particularly for Windows.
WCF or Windows Communication Foundation is a framework for building serviceoriented applications. It allows developers to build secure reliable transacted solutions that integrate across platforms and interoperate with existing investments.
Web API is a framework for building HTTP services that can be consumed by a broad range of clients including browsers mobiles iphone and tablets. Its an ideal platform for building RESTful applications on the .NET Framework.
All these technologies are widely used in the industry and have a large community of developers. They are powerful tools for building a variety of applications from small web services to large complex business applications.