Creating class libraries enables us to reuse and organize code in a way that keeps code from being dependant on other code, essentially keeping code that doesn't need to know about other code separated. The first video below covers how to edit solution and project files in Visual Studio.
The second video below covers how the different files and file types interact with each other in Visual Studio.
The Lynda video below covers how namespaces effect how the application interatcs with other projects. When a class is moved to another project the namespace of that class needs to change to match the project name. Whenever another project or class needs to use something in a project, an assembly reference and using statement need to be added.