Categories allow you to classify templates by type. You can use them to organize yourself better if you plan to use the same templates often.
To add a new category, click on Campaigns > Manage Templates > Manage Categories > Add Category. As you add templates, this view will let you see how many templates a category contains and will also allow you to delete categories.
If you wish to upload a template for a one time use, you can do so from within the Email Campaign Builder.