A collectible is one of a series of items that have a common theme. It is part of a set. Physical trading cards and stamps are examples of traditional collectibles. Collectibles are not art and artworks are not collectibles. Someone may like to collect an artist’s work or a specific style of art, but that does not make them collectibles.
Digital collectibles are a reasonably new trend. Some NFT marketplaces regard everything as collectible, so viewing this category on a marketplace may display just about anything. A collectible may appreciate in value because of its rarity or popularity. Then again, it may not.
Profile pictures (PFP) are currently popular digital collectibles. They are avatars that have been programmatically generated by an algorithm and one set may consist of up to 10,000 that have been generated from a defined number of characteristics. A set may be based on the same cute animal but what it is wearing will differ from image to image. For example, an image may have a different hat, facial expression, or items of clothing.