Below you can find simplified data model diagram for entities which can be retrieved and created using Rarible API.

Collection is a collection of NFTs. In EVMs usually collection is a smart-contract, but in some cases collection can combine several smart-contracts or vice versa - smart-contract can be divided into collections (ArtBlocks contract for example)
Item - is an entity which represents NFTs in Rarible Protocol
Ownership - is an entity which represents ownership of NFT. It's a relation between an Item and a Wallet
Order - is an entity which represents willingness to sell or buy an NFT.
Activity represents events which happen in the NFT state: mints, transfers, listings, purchases etc