GridLines is an option for two-dimensional graphics functions that specifies grid lines.
The following settings can be given for GridLines:
With the Automatic setting, grid lines are usually placed at points whose coordinates have the minimum number of digits in their decimal representation.
For each direction, the following grid line options can be given:
Grid line styles can involve graphics directives such as RGBColor and Thickness.
Grid lines are by default colored light blue.
The grid line function func[xmin, xmax] may return any other grid line option.
AbsoluteOptions gives the explicit form of GridLines specifications when Automatic settings are used.
See The Mathematica Book: Section 1.9.3, Section 2.9.5 and Section 2.9.5.
See also: Ticks, FrameTicks, FaceGrids, ColumnLines, RowLines.