Solution Explorer

A solution can include one or more projects in many different langauges.
The window can list general code modules, possibly one or more form modules or or more class modules plus a section called Reference
The last section lists the assemblies or components referenced by this particular solution so their classes become accessible

A set of projects that are part of the same application in Visual Studio .NET is called a solution. The Solution Explorer shows you a tree view list of each project, there references and components. Components can be forms, classes, modules and any other file types needed to create your application. Double click on an item to edit it within the IDE

View Code - Displays the code for the file that has the focus
View Designer - Displays the designer for the file that has the focus
Refresh - Refreshes the Solution Explorer window
Show all files - Displays all files, including any code behind form files. Project specific and is not saved.
Properties - Displays the properties for the file that has the focus

Show All Files

Adding Existing Files - Windows Forms

Select just the ".cs" file
If you are working in a Class Library you need to also add the following two references

System.Windows.Forms.dll 
System.Drawing.dll 

File Extensions