Before developing any applications, an understanding of the differences between a solution and a project is required. A project holds source files and any other files, such as images, that are needed to create an assembly that is to be run by the .NET Common Language Runtime. All of the source files in a project are compiled together to create a single assembly. The assembly may be an executable (.exe) or a dynamic link library (.dll).
A solution can hold one or more projects, but often contains just a single project. If this is the case, then there is really no difference between a solution and a project. A solution that contains more than one project is suitable for large applications that are developed by more than one person. This allows individuals or small groups of developers to independently work on the projects that make up the solution.
When you are ready to begin coding, the first step is to set up a project. The project contains all the raw materials for your application, such as the source code files, resource files such as icons, references to external files that your program depends on, and configuration data such as compiler settings. When you build a project, Visual C# invokes the C# compiler and other internal tools to create an executable assembly by using the files in your project.
参考：http://msdn.microsoft.com/en-us/library/ms173077(v=vs.90).aspx