The following image shows the Delete Category page.
The drop-down list shows all the categories listed in your database. Choose the one you wish to delete and click "Delete Category."
If the category is empty, you will see the following screen.
This category can be safely deleted because there re no images to deal with. Click "Delete Category" to permanently remove the category.
If the category has images you will be presented with this screen.
Before you can delete this category you must move the images to a new category, so they are not orphaned. Choose the category to move the images to.
You will then be presented with a summary screen.