CellGroupData[, , ... , Open] represents an open group of cells in a notebook.
CellGroupData[, , ... , Closed] represents a closed group of cells.
When a group of cells is closed, only the first member of the group is visible.
When cells are entered into a notebook, they are automatically placed in groups unless CellGrouping->Manual is set.
See The Mathematica Book: Section 2.10.7.
See also: Cell, CellGrouping, CellOpen.