Documentation

Mathematica Teacher's Edition

Built-in Functions

Notebooks

Basic Objects

Cell

Cell[contents, "style"] represents a cell in a Mathematica TE notebook.

A Mathematica TE notebook consists of a list of cells.
You can see the form of a cell as an expression by using the Show Expression command in the standard Mathematica TE front end.
You can access cells in a notebook directly using the front end.
The contents of cells can be the following:

In any given notebook, a collection of possible cell styles are defined, typically with names such as "Title", "Section", "Input" and "Output".
Cells can have many options, including:

See The Mathematica Book on the web: Section 2.10.1 and Section 2.10.7.