What's the difference between a collection and a category

A collection is a hand-picked group you build by tapping products — any products, for any reason (Summer, Sale, Bestsellers) — and fully control here: its own cover, name and "Shown in store" / "Hidden" state. A category is a single label you give each product when adding it, describing what kind of thing it is. Use a collection for curated or marketing groupings; use the category for what type of product it is. Gotcha: one product can sit in many collections at once, but its category is a separate label on the product — they are not the same list.