Backface Culling:
    If this box is checked, OpenGL won't draw polygons that are facing away from you (that is, their normal vector is pointing into the screen rather than out towards you).

Show Subdivision Cages:
    If this box is checked, the outer cage of a subdivided shape will always show.  If it is unchecked, you will only see the subdivided shape in every mode except for object selection mode, and you can make your selection and edits directly from the subdivided surface (the "iso surface").

Show Cage Selections:
    If this option is checked and Show Subdivision Cages is unchecked, on subdivided shapes you will only see the portion of the control cage which is currently selected (i.e. the particular edges, faces, or vertices that are currently selected).

Line Width:
    This field allows you to change the width in pixels of all geometry lines. This will not affect the manipulator or grid lines.

Vertex Size:
    This field allows you to change the size in pixels of all vertices.

Normals Length:
    When normals are displayed, they are displayed at a fixed length. This field allows you to change that length.

Walkthrough Speed:
    In Walkthrough Mode (Display > Walkthrough Mode), you may find that the camera zooms and pans at an undesirable speed. This field allows you to change the speed.

Camera FOV:
    The perspective camera's FOV (field of view) is the angle in degrees which determines how much of the scene the camera can see at once. Large angles will result in a "fisheye" effect.

Near Clip Dist:
    The near clip distance is the distance from the camera which geometry has to be in order for it to be displayed in the viewport. Values must be larger than 0 and not equal to 0 for the display to function properly.

Far Clip Dist:
    The far clip distance is the distance from the camera at which an object ceases to be displayed. In a large scene, this can speed up performance by not displaying irrelevant geometry.

Manipulator Size:
    This field allows you to change the size, in pixels, of the manipulator. Changing this value will also update the size of the alternate manipulator accordingly.

Grid Spacing:
    This field allows you to change the spacing between grid lines. This change affects all work grids.

Grid Width :
    This field allows you to change the overall width of the grid. This value refers to the distance from the origin to each side of the grid. (So a grid with width 10m will be 10m in the positive X direction as well as 10m in the negative X direction.)

Heavy Line Spacing :
    This field allows you to set how many lines of the grid to skip before making a thick line.  This allows you to, for example, have the grid spacing set at 1 inch and set the heavy line spacing to 12 so that every foot (i.e. every 12 inches) is marked with a heavy line while inches are marked with a lighter line.