The 3D View is where you perform most of the object modeling and scene creation. Blender has a wide array of tools and options to support you in efficiently working with your mouse, keyboard and keypad. Your flat (two-dimensional) monitor is your viewport into the 3D space.
Blender is able to "reach in" to other .blend files and pull in whatever you want. In this way, Blender supports reuse of your graphical models. For example, if you have a library .blend file that has a really neat Material used in it, you can, from your current .blend file, Append that Material into your current .blend file. This saves you from manually re-creating all the different settings.
The Preview panel gives a quick visualization of the active material and its properties, including its Shaders, Ramps, Mirror Transp properties and Textures. It provides many useful shapes that are very useful for designing new shaders: for some shaders (like those based Ramp colors or a Diffuse shader like Minnaert), one needs fairly complex or specific previewing shapes to decide if the shader-in-design achieves its goal.