The WIN32 API is a set of libraries that allow you to interact with windows. In C#, you shouldn't need to use them very often (although that is not to say never!). This is because C# utilizes the .Net framework - which wraps a lot of the WIN32 API functions in managed classes. To use the WIN32 API in C# you will need to reference them as an external dll import. Googling 'dll import win32 api c#' would be a good place to start learning.