The document explains what a web API is, specifically focusing on ASP.NET Web API, which is an extensible framework for creating HTTP-based services accessible across various applications and platforms. It outlines the characteristics, supported data formats, and the differences between ASP.NET Web API and WCF, particularly in terms of protocol support and application scenarios. Additionally, it advises when to choose ASP.NET Web API over WCF, depending on the .NET framework version and the need for RESTful services.