a table for categories, a table for products, then a third table, the relationship table, which associates products and categories
there can be multiple rows in the third table for a single product, and multiple rows for a single category
this third table will have only two columns, the product_id and the category_id, each of which is a foreign key to its respective table
together, the two columns form a composite primary key (so that you cannot put the same product into the same category moire than once)