Library card catalogs made it possible for users to search for items by title, author, and subjects. These same fields - and many more - are the basis of how we make items searchable in databases.
On the card below, you can see that the user would have been able to find this item by looking up the title (Medical nursing for animal health technicians), author (Pratt, Paul), or subjects Veterinary nursing or Veterinary medicine.
Instagram uses a method of organizing information that is more informal, but very similar, to library card catalogs. Users can search for photos by accounts either posted or tagged in posts, by location markers, or hashtags (informal subject headings).
Image source: Instagram @NatGeo