Notebooks as Documents

Wolfram System notebooks allow you to create documents that can be viewed interactively on screen or printed on paper.
Particularly in larger notebooks, it is common to have chapters, sections and so on, each represented by groups of cells. The extent of these groups is indicated by a bracket on the right.
The grouping of cells in a notebook is indicated by nested brackets on the right.

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A group of cells can be either open or closed. When it is open, you can see all the cells in it explicitly. But when it is closed, you see only the cell around which the group is closed. Cell groups are typically closed around the first or heading cell in the group, but you can close a group around any cell in that group.
Large notebooks are often distributed with many closed groups of cells, so that when you first look at the notebook, you see just an outline of its contents. You can then open parts you are interested in by doubleclicking the appropriate brackets.
Doubleclicking the bracket that spans a group of cells closes the group, leaving only the first cell visible.

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When a group is closed, the bracket for it has an arrow at the bottom. Doubleclicking this arrow opens the group again.

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Doubleclicking the bracket of a cell that is not the first of a cell group closes the cell group around that cell and creates a bracket with up and down arrows (or only an up arrow if the cell was the last in the group).

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Each cell within a notebook is assigned a particular style which indicates its role within the notebook. Thus, for example, material intended as input to be executed by the Wolfram Language kernel is typically in Input style, while text that is intended purely to be read is typically in Text style.
The Wolfram System front end provides menus and keyboard shortcuts for creating cells with different styles, and for changing styles of existing cells.
This shows cells in various styles. The styles define not only the format of the cell contents, but also their placement and spacing.

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By putting a cell in a particular style, you specify a whole collection of properties for the cell, including for example how large and in what font text should be given.
The Wolfram System front end allows you to modify such properties, either for complete cells, or for specific material within cells.
Even within a cell of a particular style, the Wolfram System front end allows a wide range of properties to be modified separately.

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Ordinary Wolfram System notebooks can be read by non-Wolfram System users using the free product, Wolfram Player, which allows viewing and printing but does not allow computations of any kind to be performed. This product also supports interaction with dynamic content from the output of Manipulate. For example, all the notebook content on the Wolfram Demonstrations Project site is available as notebook player files.
Wolfram System front end
creating and editing Wolfram System notebooks
Wolfram Language kernel
doing computations in notebooks
Wolfram Player
reading .nb and .cdf files
Programs required for different kinds of operations with notebooks.