CellTags is an option for Cell which gives a list of tags to associate with a cell.
Cell tags are typically used to allow searching for cells.
The tags are usually strings.
Cell tags are displayed when the setting ShowCellTags->True is made.
See The Mathematica Book: Section 2.10.1, Section 2.10.3 and Section 2.10.9.
See also: CellLabel, ConversionRules.