Building creates the application's .dll and .idy files in the New_Configuration.bin subfolder of your project.
By default Eclipse builds your project automatically whenever you save a file. If you haven't configured Eclipse to do this, you can build a project at any time by selecting the project, then on the Project menu, select Build Project; you can also select Build All to build all projects in the current workspace, or Build Working Set to build a subset of the projects in the workspace. To turn off automatic building, select Window > Preferences > General > Workspace and clear the Build automatically check box.