ColumnAlignments is an option for GridBox which specifies how entries in each column should be aligned.
The following settings can be given:
Lists of settings are used cyclically if there are more columns in the grid than elements in the list.
With the setting ColumnAlignments->"c" a column will be right justified if the character c appears nowhere in it.
You can insert invisible \[AlignmentMarker] characters in the entries in a grid to specify how these entries should be aligned.
See The Mathematica Book: Section 2.8.11.
See also: RowAlignments, ColumnsEqual, TableAlignments, TextAlignment.