Content in a Wolfram System notebook is organized in cells. Each cell has a cell bracket that appears along the right edge of the notebook window. Markings on a cell bracket indicate important information about that cell.
If you just start typing in a typical Wolfram System notebook, you will see a cell bracket with a triangular notch at the top. These cells can contain typeset Wolfram Language expressions and can be evaluated:
A cell can also be marked as an initialization cell. Initialization cells are evaluated automatically before you evaluate other cells in the notebook. An initialization cell has a short vertical bar at the top of its cell bracket:
A cell with a dashed line down the whole height of the cell bracket is a TraditionalForm cell. TraditionalForm cells contain typeset formulas that might be ambiguous if they were to be interpreted as Wolfram Language expressions:
Evaluating a cell that contains a syntax error will add an error indication to the cell bracket, indicating a problem with the cell's content. You can click on the indicator to show the error messages, as was done in this screen shot:
Other cell bracket annotations include the letter A to indicate that a cell is active, the letter X to indicate that editing is not allowed in that cell, or a filled square to indicate that the CellSize option is set.
Cell groups can be closed by double-clicking the grouping bracket. The grouping bracket here was double-clicked to close the group. The direction of the arrow that appears on the grouping bracket indicates the direction of any hidden cells:
Double-clicking the bracket of an output cell will hide the input cell that produced it. This is useful if you want to keep the input for future use or reference, but do not need to view it all the time.
Here, the bracket of the output cell containing 3 was double-clicked to hide the input cell containing 1+2. Note the arrow on the grouping bracket points up to indicate the direction of the hidden input cell: