Is a Library a project or a C source code file in Visual Studio

Question

Certain things in Visual Studio have a tendency to be equivalent. When a project is compiled, for example, an assembly (exe and dll file) is created. As a result, when the project is compiled, Project equals Assembly.

Is a Library a Project, a single C# source file, or a collection of C# source files in Visual Studio? Maybe it's something I didn't think about.