Zettelkästen is German for "note squares" or "note boxes". This note is an example of a "Zettel".
Niklas Luhmann was a German who is famous for using Zettelkästen to organize his thoughts.
The Zettelkästen method is a way of taking notes that is recognized by pieces of information, often brief, that refer to other pieces in an interlinked system. It is similar to the concept of a memex or personal wiki, but dividing Zettels into categories is discouraged.