A tax is a rate you attach to products, such as VAT at 20%.
Tax is included in the price, not added on top. A product priced at 100.00 with a 20% tax includes 16.67 tax.
Attach a tax to a product with product_tax_id.
To stop offering a tax, set active to false. The dashboard then leaves it out of its pickers. The API still accepts it on products and orders, so remove it from your products too.