HS codes are the abbreviation of Harmonized System. These codes designate products and product categories according to a 2 to 6 digit nomenclature:
The first 2 digits of the code indicate the product category
The next 4 to 6 digits indicate the subcategories in which the product falls
Countries may add digits to designate the products in a more precise way; the code has a limit of 10 digits. The 8 or 10 digit HS code used by a country is called a "tariff line".
You can determine the HS code of your goods by consulting the Customs Tariff document or through this interactive tool.