Typical stylesheets make the Wolfram System automatically organize notebook cells into convenient groups. You can override this behavior, or control it in any way, using menu items, option settings, or from within a program.
Group Cells (Ctrl+G) — group together selected cells
Ungroup Cells (Ctrl+U) — ungroup selected cells
CellGrouping — whether to do automatic grouping of cells in a notebook
CellGroupingRules — precedence and grouping type for cells
ShowGroupOpener — whether to control cell groups with
CellGroup — symbolic representation of a cell group within a notebook expression