Categories can be archived to remove categories from your account, while having the option to restore them later on. When archived and restored, all of the historical information will remain.
To archive a category, simply click on the box icon to the right of your category on your dashboard.
To restore a category, simply click on the "Restore Archived Categories" button at the bottom of the dashboard then the green icon to restore on the left to restore it. The "Restore Archived Categories" button will only appear if there are any archived categories.