Unable to find the product category . How do get the product category?

Shopify Partner
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I am new to ecommerce platform .  Now i am creating a shopify publilc APP. I have a confussion to finalize the product category. In that App i need to get the product category.

For that i search in google about shopify product category but  i am not able to find that. I found API for Collection . I think collection is a category. If i am wrong please provide clarification ?

If collection is the category means , i have an another doubt, i can create a product without collection also. is this a correct process? A product without a category is acceptable? . is any default collection is assigned for those product?

Please guide me how to get the product category and clarify what is the collection in shopify