Rectangle[xmin, ymin, xmax, ymax] is a two-dimensional graphics primitive that represents a filled rectangle, oriented parallel to the axes.
Rectangle[xmin, ymin, xmax, ymax, graphics] gives a rectangle filled with the specified graphics.
Scaled and Offset can be used to specify the coordinates for the rectangle.
Rectangle[Scaled[xmin, ymin], Scaled[xmax, ymax]] yields a rectangle with corners specified by scaled coordinates.
Any combination of ordinary coordinates, as well as Scaled and Offset, can be used to specify the corners of the rectangle.
Rectangle[xmin, ymin, xmax, ymax] is equivalent to a suitable Polygon with four corners.
You can use graphics directives such as GrayLevel and RGBColor to specify how Rectangle[xmin, ymin, xmax, ymax] should be filled.
In Rectangle[xmin, ymin, xmax, ymax, graphics], graphics can be any graphics object.
The rectangle is taken as the complete display area in which the graphics object is rendered.
When rectangles overlap, their backgrounds are effectively taken to be transparent.
Fonts and absolute size specifications are not affected by the size of the rectangle in which the graphics are rendered.
The options DisplayFunction, ColorOutput and CharacterEncoding are ignored for graphics objects given inside Rectangle.
See The Mathematica Book: Section 2.9.2.
See also: Polygon, Raster, RasterArray, Cuboid, GraphicsArray.