When you first start Mathematica, it displays an empty notebook with a blinking cursor. You can start typing right away. Notice the bracket that appears along the right side of the notebook in line with what you just typed. This bracket shows the extent of the cell:
By default, new cells are in the Input cell style. The contents of Input cells are interpreted as commands that can be evaluated using Shift+Enter. The result is displayed in a separate Output cell following the Input cell, as indicated by the labels on the left after evaluating:
The default style of cells is Input; instead, you can create a cell containing text to add comments or descriptions of your programs. With an insertion point indicated by the horizontal cell insertion bar, create a new cell in the Text cell style from the Format ► Style menu, or use the keyboard shortcut indicated in the menu.
You can change the style of a cell by selecting its cell bracket on the right and then choosing a different cell style from the Format ► Style menu. The Input cell here is changed to the Section cell style:
Other information that also can be edited from the Format menu includes font color, size, and font.
Some cells have automatic hierarchical grouping behavior. For example, Section cells group all Subsection and Text cells. The layering of brackets on the right side of the notebook indicates grouping. Grouping allows you to organize the contents of a notebook.
The closing behavior of input/output groups is slightly different. In addition to closing such a group by double-clicking its cell bracket, you can do so by double-clicking the cell bracket of just the Input cell:
Double-clicking the cell bracket of just the Output cell in an input/output group will close the group around that Output cell. This is a convenient way to display the result of an evaluation without displaying the command that produced it, while keeping access to the input for later changes.