The categories you add to your listings should describe what your product is, but adding a few descriptive tags lets you describe your product in your own words. Remember: Multi-word phrases are better than individual descriptive words.
Examples: 1920s cat brooch, reusable straw pouch, striped ceramic mug, set of four coasters
USEO offers a tool for research of Etsy top search keywords based on real statistics for your shop.