A way to format a Gantt chart's bars to emphasize attributes including; critical tasks, milestones, summary tasks, and slack.
Text can also be added to the bars.
One set of styles is associated with each chart-type view. The view's parameters (including bar styles) are saved with the current project document.
Set by the Format > Bar Styles command or fast-formatted using the Gantt Chart Wizard.
SOURCE: microsoft project glossary-projectlearning.net