The Window menu contains a list of all the open primary windows, and lets you switch between them by selecting them in the menu. Primary windows are windows like the main window, Render window and any View windows containing 3D Previews, shader previews and network views. This does not include any floating windows like node parameter floaters or the 3D Preview Location window.
The Show/Hide Floating Windows item allows you to quickly show and hide any floating windows, such as node parameter floaters or the 3D Preview Location window.
In the Mac version there is one other item, Close Window, which is the first item in the menu and can be used to close the frontmost window.
A shader is a program or set of instructions used in 3D computer graphics to determine the final surface properties of an object or image. This can include arbitrarily complex descriptions of light absorption and diffusion, texture mapping, reflection and refraction, shadowing, surface displacement and post-processing effects. In Terragen 2 shaders are used to construct and modify almost every element of a scene.
A parameter is an individual setting in a node parameter view which controls some aspect of the node.