In: knowledge. Also: naming.
Backlinks: Naming.

Classification

A system for classifying (distinguishing and distribution of) things into groups that share similar qualities.

Q: Why is this important to me?
A: Because I want to organize my memex pages and I'm curious how other general knowledge databases classify their main topics.

Entry points

Wikipedia's main articles

https://wikipedia.org/wiki/Wikipedia:Contents/Categories
https://wikipedia.org/wiki/Category:Main_topic_articles
https://wikipedia.org/wiki/Category:Main_topic_classifications

Culture and arts
General reference
Geography and places
Health and fitness
History and events
Human activities
Mathematics and logic
Natural and physical sciences
People and self
Philosophy and thinking
Religion and belief systems
Society and social sciences
Technology and applied sciences

Main topic articles:

Academic discipline
Business
Concept
Culture
Economy
Education
Energy
Engineering
Entertainment
Ethics
Event (philosophy)
Food and drink
Geography
Government
Health
History
Humanities
Human nature
Information
Knowledge
Language
Law
Life
Mass media
Mathematics
Military
Music
Nature
Non-physical entity
Person
Philosophy
Policy
Politics
Religion
Science and technology
Society
Sport
Universe
World

Main topic classifications:

Academic disciplines
Business
Concepts
Culture
Economy
Education
Energy
Engineering
Entertainment
Entities
Ethics
Events
Food and drink
Geography
Government
Health
History
Humanities
Human nature
Information
Knowledge
Language
Law
Life
Mass media
Mathematics
Military
Music
Nature
People
Philosophy
Policy
Politics
Religion
Science and technology
Society
Sports
Universe
World

Citizendium

Citizendium is the closest thing to Wikipedia I could find, so of course I had to see how they organize their main categories:

Agriculture
Anthropology
Archaeology
Architecture
Astronomy
Biology
Business
Chemistry
Classics
Computers
Earth Sciences
Economics
Education
Engineering
Food Science
Games
Geography
Health Sciences
History
Hobbies
Journalism
Law
Library and Information Science
Linguistics
Literature
Mathematics
Media
Military
Music
Philosophy
Physics
Politics
Psychology
Religion
Robotics
Sociology
Sports
Theater
Topic Informant
Visual Arts

Usenet

https://wikipedia.org/wiki/Alt.*_hierarchy
https://wikipedia.org/wiki/Great_Renaming

comp.* – computer-related discussions (comp.software, comp.sys.amiga)
humanities.* – fine arts, literature, and philosophy (humanities.classics, humanities.design.misc)
misc.* – miscellaneous topics (misc.education, misc.forsale, misc.kids)
news.* – discussions and announcements about news (meaning Usenet, not current events) (news.groups, news.admin)
rec.* – recreation and entertainment (rec.music, rec.arts.movies)
sci.* – science related discussions (sci.psychology, sci.research)
soc.* – social discussions (soc.college.org, soc.culture.african)
talk.* – talk about various controversial topics (talk.religion, talk.politics, talk.origins)

Groups in the alt.* hierarchy tend to be more specialized or specific.
Companies and projects admin their own hierarchies to discuss their products and offer community technical support, such as the historical gnu.* hierarchy.
Regional and language-specific hierarchies such as japan.*, malta.* and ne.* serve specific countries and regions.
Informal sub-hierarchy conventions also exist. *.answers are typically moderated cross-post groups for FAQs.

Encyclopædia Britannica

Entertainment & Pop Culture
Geography & Travel
Health & Medicine
Lifestyles & Social Issues
Literature
Philosophy & Religion
Politics, Law & Government
Science
Sports & Recreation
Technology
Visual Arts
World History

HowStuffWorks

https://howstuffworks.com/about-hsw.htm

Adventure
Animals
Auto
Culture
Entertainment
Health
Home & Garden
Lifestyle
Money
Quizzes
Reviews
Science
Tech

BBC

Culture
Future
Music
News
Reel
Sounds
Sport
Travel
TV
Weather
Worklife

News:

Business
Entertainment & Arts
In pictures
Long reads
Newsbeat
Reality check
Science
Tech
Video
World
World news TV

CNN

Business
Entertainment
Features
Health
Investigations
Longform
Photos
Politics
Sport
Style
Tech
Travel
Video
World