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 Wolfram System input cell.

1.gif

Here are the expressions corresponding to these cells.

2.gif

Here is a notebook containing a text cell with the Wolfram System input inside.

3.gif

This is the expression corresponding to the cell. The Wolfram System input is in a cell embedded inside the text.

4.gif

"text"plain text
TextData[{text1,text2,}]text potentially in different styles, or containing cells
BoxData[boxes]formatted Wolfram System 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.