When you're viewing the projects list, cards or scheduler view. Or viewing the tasks list or scheduler view. You have the option to "Group by" using the button on the top row:

When you click one of these options the page will group all items and show a header with the group names. Like this:

This is a great way to see all items categorised by a particular grouping. This can be particularly useful when you're on the tasks scheduler view as it will group all items down the left and show you tasks on the scheduler timeline:

You can group by different things depending on which view you're on.

You can also group by custom field for most views. If you create a custom field then choose to group by it so that you can see items grouped by your custom data points.