Zettelkasten is a knowledge management and note-taking method used by students, writers and researchers.
It consists of taking notes and relationships between them.
For example, Niklas Luhmann wrote 90.000 cards and 70 books. He wrote these short notes about many of his multiple interests before he started writing books. Then, he published books really fast as he already had interconnected notes, written in his own words which he could reuse.
Until now, it has been mainly used in private where writers and researchers keep their notes and only publish scientific papers, books, articles, etc. — not the notes themselves.