This is documentation for Mathematica 8, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.1)

The Structure of Cells

Cell[contents,"style"]a cell in a particular style
Cell[contents,"style1","style2",...]a cell with multiple styles
Cell[contents,"style",options]a cell with additional options set

Expressions corresponding to cells.

Here is a notebook containing a text cell and a Mathematica input cell.
Here are the expressions corresponding to these cells.
Here is a notebook containing a text cell with Mathematica input inside.
This is the expression corresponding to the cell. The Mathematica input is in a cell embedded inside the text.
"text"plain text
TextData[{text1,text2,...}]text potentially in different styles, or containing cells
BoxData[boxes]formatted Mathematica expressions
GraphicsData["type",data]graphics or sounds
OutputFormData["itext","otext"]text as generated by InputForm and OutputForm
RawData["data"]unformatted expressions as obtained using Show Expression
CellGroupData[{cell1,cell2,...},Open]an open group of cells
CellGroupData[{cell1,cell2,...},Closed]a closed group of cells
StyleData["style"]a style definition cell

Expressions representing possible forms of cell contents.