up:

In the history of electronical information organization systems there have been essentially two popular approaches:

    1. Data is organized into trees, which itself have other trees as children. Every item appears only once in the structure. This is a very strict and unflexible way to organize information, and inherits limitations of the physical world.

    2. Systems that use hierarchies are for example file systems (without hard links) or outliners.

    1. Every item is autonomous and connected to other items by the use of links. This is a very flexible way to organization, which again is problematic since networks like these often tend to lack any structure, red threads and appear like a mess.

    2. Systems with chaotic structure are the Web, Wikis and most other hypertext systems.