Swagger is one of the most popular documentation and code generation engine for Web API. Unlike ASMX WebServices where we could use WSDL to generate C# class file to refer and write code which could compile we need some client wrapper in Web API. Hence Swagger comes handy to perform those documentation and client code generation with a strongly typed code base.
For more details about Swagger please visit http://swagger.io/
There is a nice blog about how you use Swagger with ASP.NET Web API http://www.wmpratt.com/swagger-and-asp-net-web-api-part-1/