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.
We'd like to give you a quick tour of the startup windows and the menus, to give you a sneak peek at Blender's makeover. Blender's 2.5 new look is very exciting.