RowLines is an option for GridBox which specifies whether lines should be drawn between adjacent rows.
The default setting is RowLines->False.
RowLines->, , ... specifies whether lines should be drawn between successive pairs of rows. The can be True or False.
If there are more rows than entries in the list, the last element is used repeatedly for remaining pairs of rows.
Lines can be drawn around the outside of a GridBox using FrameBox.
See The Mathematica Book: Section 2.8.10.
See also: ColumnLines, FrameBox, GridLines.