Show[graphics, options] displays two- and three-dimensional graphics, using the options specified.
Show[, , ... ] shows several plots combined.
Show can be used with Graphics, Graphics3D, SurfaceGraphics, ContourGraphics, DensityGraphics and GraphicsArray.
Options explicitly specified in Show override those included in the graphics expression.
When plots are combined, their lists of non-default options are concatenated.
Show is effectively the analog of Print for graphics. The option DisplayFunction determines the actual output mechanism used.
Functions like Plot automatically apply Show to the graphics expressions they generate.
See The Mathematica Book: Section 1.9.4 and Section 2.9.1.
See also: Plot, etc., and Display.